[onechat] Introduce plugin and tool registry (#220889)

## Summary

Implements the onechat tool registry RFC.

Fix https://github.com/elastic/search-team/issues/9938
Fix https://github.com/elastic/search-team/issues/10019

This PR introduces the following artifacts:

**plugins:**
- `onechat`

**packages:**
- `@kbn/onechat-common`
- `@kbn/onechat-server`
- `@kbn/onechat-browser`

## Tool APIs overview

### Registering a tool

```ts
class MyPlugin {
  setup(core: CoreSetup, { onechat }: { onechat: OnechatPluginSetup }) {
    onechat.tools.register({
      id: 'my_tool',
      name: 'My Tool',
      description: 'My very first tool',
      meta: {
        tags: ['foo', 'bar'],
      },
      schema: z.object({
        someNumber: z.number().describe('Some random number'),
      }),
      handler: ({ someNumber }, context) => {
        return 42 + someNumber;
      },
    });
  }
}
```

### Executing a tool

Using the `execute` API:

```ts
const { result } = await onechat.tools.execute({
  toolId: 'my_tool',
  toolParams: { someNumber: 9000 },
  request,
});
```

Using a tool descriptor:

```ts
const tool = await onechat.tools.registry.get({ toolId: 'my_tool', request });
const { result } = await tool.execute({ toolParams: { someNumber: 9000 } });
```

With error handling:

```ts
import { isToolNotFoundError } from '@kbn/onechat-common';

try {
  const { result } = await onechat.tools.execute({
    toolId: 'my_tool',
    toolParams: { someNumber: 9000 },
    request,
  });
} catch (e) {
  if (isToolNotFoundError(e)) {
    throw new Error(`run ${e.meta.runId} failed because tool was not found`);
  }
}
```

### Listing tools

```ts
const tools = await onechat.tools.registry.list({ request });
```

*More details and example in the plugin's readme.*

### What is **not** included in this PR:

- tool access control / authorization - we have a dedicated RFC
- dynamic tool registration / permissions checks part/of depends on the
authorization RFC
- feature / capabilities - will come with browser-side and HTTP APIs
- fully defining tool meta - hard to do now
- filter parameters for the tool list API - depends on the meta being
defined

*Those will be follow-ups*. Everything else from the RFC should be
there.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-optimizer/limits.yml

@@ -113,6 +113,7 @@ pageLoadAssetSize:
   observabilityLogsExplorer: 46650
   observabilityOnboarding: 19573
   observabilityShared: 111036
+  onechat: 25000
   osquery: 107090
   painlessLab: 179748
   presentationPanel: 11550