Resolves https://github.com/elastic/kibana/issues/159598
## Summary
This PR modifies the `test` subaction of the Generative AI connector to
accept a `stream` parameter (default: `false`) that allows for a
streaming response.
The Generative AI connector is basically a pass-through to the Open
AI/Azure OpenAI APIs, where the `stream` parameter is passed in via the
body of the request. This means that with the existing connector, users
could specify `stream: true` in the body which would lead to unexpected
results when the action is unprepared to return streaming results. This
PR sanitizes the body that is passed in the `run` subaction to prevent
the `stream` parameter from being set to `true` and explicitly sets the
`stream` parameter for the `test` subaction.
In order to test the streaming response, I created an example plugin
that prompts users to create a Generative AI connector if one does not
exist and then executes actions using the connector with `stream` set to
`true`. This borrows liberally from @dgieselaar's existing work from
https://github.com/elastic/kibana/pull/158678441694cb-0154-4450-bd93-3907c4a9995c
## To Verify
1. Navigate to https://localhost:5601/app/GenAiStreamingResponseExample
2. Set up a Generative AI connector
3. Open the network console. Enter a prompt and click `Stream Response`
4. You should see the chat response return streaming results.
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Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
