[SIEM][Detection Engine] Critical blocker, adds need REST prefix for cloud

## Summary

* Adds needed `/` to the beginning of cloud requests from Kibana -> ES

### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

~~- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)~~

~~- [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)~~

~~- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~~

~~- [ ] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios~~

~~- [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~~

### For maintainers

~~- [ ] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

- [x] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)

x-pack/legacy/plugins/siem/server/lib/detection_engine/index/create_bootstrap_index.ts

@@ -19,7 +19,7 @@ export const createBootstrapIndex = async (
   index: string
 ): Promise<unknown> => {
   return callWithRequest('transport.request', {
-    path: `${index}-000001`,
+    path: `/${index}-000001`,
     method: 'PUT',
     body: {
       aliases: {

x-pack/legacy/plugins/siem/server/lib/detection_engine/index/delete_policy.ts

@@ -11,7 +11,7 @@ export const deletePolicy = async (
   policy: string
 ): Promise<unknown> => {
   return callWithRequest('transport.request', {
-    path: `_ilm/policy/${policy}`,
+    path: `/_ilm/policy/${policy}`,
     method: 'DELETE',
   });
 };

x-pack/legacy/plugins/siem/server/lib/detection_engine/index/get_policy_exists.ts

@@ -12,7 +12,7 @@ export const getPolicyExists = async (
 ): Promise<boolean> => {
   try {
     await callWithRequest('transport.request', {
-      path: `_ilm/policy/${policy}`,
+      path: `/_ilm/policy/${policy}`,
       method: 'GET',
     });
     // Return true that there exists a policy which is not 404 or some error

x-pack/legacy/plugins/siem/server/lib/detection_engine/index/set_policy.ts

@@ -12,7 +12,7 @@ export const setPolicy = async (
   body: unknown
 ): Promise<unknown> => {
   return callWithRequest('transport.request', {
-    path: `_ilm/policy/${policy}`,
+    path: `/_ilm/policy/${policy}`,
     method: 'PUT',
     body,
   });

x-pack/legacy/plugins/siem/server/lib/detection_engine/privileges/read_privileges.ts

@@ -11,7 +11,7 @@ export const readPrivileges = async (
   index: string
 ): Promise<unknown> => {
   return callWithRequest('transport.request', {
-    path: `_security/user/_has_privileges`,
+    path: '/_security/user/_has_privileges',
     method: 'POST',
     body: {
       cluster: [