[Meta][8.x] Kibana AI Connector integration with GenAI components

[YulNaumenko]

For 8.x the decision is to introduce pre-configured Kibana AI Connector which will be integrated with EIS service provider exposing Elastic default LLM.
TODO list for getting to the goal of integrating Elastic default LLM within EIS and Inference API into Kibana GenAI functionality as a default experience:

• Introduce EIS service in the Kibana AI Connector providers list (hardcoded?)
• Merge PR for _unified completion task implemented for EIS provider [Inference API] Add unified api for chat completions elasticsearch#117589
• Implement Kibana AI Connector sub action to support new _unified completion task [AI Connector] Kibana inference connector should support openai compatible schema for completion task #202621
• Create pre-configured Kiabana AI Connector for ESS and Serverless
• Integrate pre-configured Kibana AI Connector to the GenAI Kibana components (Security/Observability/Search) and select it by default. Security issue [Security Solution] Integrate pre-configured Kibana AI Connector to the Security GenAI powered functionality #202625, O11y/Search [AppEx AI] Integrate pre-configured Kibana AI Connector to the O11y and Search GenAI powered functionality #202626
• Change feature flag for Kibana AI Connector to be available by default