Create new merge_request_diff_llm_summary table

Create a new table for storing responses from LLM-generate summaries of diffs.

Fields to include:

1. provider (enum of supported providers)
2. merge_request_diff_id
3. user_id (if available, the human who initiated the generation)
4. source (enum/identifier of internal service/process/worker/endpoint handling the request)
5. type (string? image? or perhaps summary, conversation, etc? 🤔)
6. timestamps
7. content Actual content of the response message

Once created, additionally create an ActiveRecord model that relates the summary to the merge_request_diff and the merge_request