[Synthtrace] Fix wrong url build in the Kibana client (#217678)

Relates to #217529 
#216653
#216844

## Summary

The issue was introduced in the [PR
here](https://github.com/elastic/kibana/pull/212120/files#diff-34f8e7299930135fd708d98018fc6f4141d6e7c25df7e5fdb90f3472ad0e2948R36):
basically, the URL will look like: `
http:/user:pass@localhost:5620/api/fleet/epm/packages/apm?prerelease=false`
because `Path.join` will strip the `/` which is needed in this case -
this URL is also passed to `getFetchAgent`. This PR will fix this issue.

src/platform/packages/shared/kbn-apm-synthtrace/src/lib/shared/base_kibana_client.ts

@@ -11,9 +11,9 @@
 
 import fetch from 'node-fetch';
 import { RequestInit } from 'node-fetch';
-import Path from 'path';
 import { kibanaHeaders } from './client_headers';
 import { getFetchAgent } from '../../cli/utils/ssl';
+import { normalizeUrl } from '../utils/normalize_url';
 
 type KibanaClientFetchOptions = RequestInit;
 
@@ -33,18 +33,20 @@ export class KibanaClient {
   }
 
   fetch<T>(pathname: string, options: KibanaClientFetchOptions): Promise<T> {
-    const url = Path.join(this.target, pathname);
-    return fetch(url, {
+    const pathnameWithLeadingSlash = pathname.startsWith('/') ? pathname : `/${pathname}`;
+    const url = new URL(`${this.target}${pathnameWithLeadingSlash}`);
+    const normalizedUrl = normalizeUrl(url.toString());
+    return fetch(normalizedUrl, {
       ...options,
       headers: {
         ...this.headers,
         ...options.headers,
       },
-      agent: getFetchAgent(url),
+      agent: getFetchAgent(normalizedUrl),
     }).then(async (response) => {
       if (response.status >= 400) {
         throw new KibanaClientHttpError(
-          `Response error for ${options.method?.toUpperCase() ?? 'GET'} ${url}`,
+          `Response error for ${options.method?.toUpperCase() ?? 'GET'} ${normalizedUrl}`,
           response.status,
           await response.json().catch((error) => {
             return undefined;

src/platform/packages/shared/kbn-apm-synthtrace/src/lib/utils/normalize_url.ts

@@ -0,0 +1,12 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+export function normalizeUrl(url: string): string {
+  return url.replace(/([^:]\/)\/+/g, '$1');
+}

src/platform/packages/shared/kbn-apm-synthtrace/src/lib/utils/normalized_url.test.ts

@@ -0,0 +1,72 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import { normalizeUrl } from './normalize_url';
+
+describe('normalizeUrl', () => {
+  it('should remove unnecessary double slashes from the URL', () => {
+    const url = 'http://example.com//some//path///to/resource';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com/some/path/to/resource');
+  });
+
+  it('should preserve the protocol slashes', () => {
+    const url = 'http:///example.com';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com');
+  });
+
+  it('should handle URLs with query parameters', () => {
+    const url = 'http://example.com//some/path?key=value&key2=value2';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com/some/path?key=value&key2=value2');
+  });
+
+  it('should handle URLs with port', () => {
+    const url = 'http://example:3000//some/path?key=value&key2=value2';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example:3000/some/path?key=value&key2=value2');
+  });
+
+  it('should handle URLs with localhost port and extra path', () => {
+    const url = 'http://localhost:3000//some/path//more/path';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://localhost:3000/some/path/more/path');
+  });
+
+  it('should handle URLs with trailing slashes', () => {
+    const url = 'http://example.com/some/path///';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com/some/path/');
+  });
+
+  it('should handle URLs without double slashes', () => {
+    const url = 'http://example.com/some/path';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com/some/path');
+  });
+
+  it('should handle URLs with no path', () => {
+    const url = 'http://example.com';
+    const result = normalizeUrl(url);
+    expect(result).toBe('http://example.com');
+  });
+
+  it('should handle empty strings', () => {
+    const url = '';
+    const result = normalizeUrl(url);
+    expect(result).toBe('');
+  });
+
+  it('should handle URLs with differnt protocol', () => {
+    const url = 'ftp://example.com//some/path';
+    const result = normalizeUrl(url);
+    expect(result).toBe('ftp://example.com/some/path');
+  });
+});