Adds new MergeRequests::Llm::SummarizeMergeRequestService

What does this MR do and why?

Adds a very basic service class for generating a summary of the merge request based upon the latest diff. The goal here is to provide support for rapidly developing experimental features.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #404901 (closed)