Add feature flag for summarize merge request model upgrade

What does this MR do and why?

• This MR adds a new feature flag, summarize_merge_request_claude_3_7_sonnet, which will be used to rollout the model upgrade to Claude 3.7 Sonnet for summarize merge request feature.
• MR that adds a new prompt for Claude 3.7 is merged: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2051 (merged)
• Relates to #521400 (closed), https://gitlab.com/gitlab-org/gitlab/-/issues/478035

How to set up and validate locally

• Create a new MR
• Click on Summarize Code Changes and make sure Claude 3.5 model is used by checking the logs in gdk tail -f gitlab-ai-gateway | grep "Request to LLM"
• Enable the feature flag and make sure Claude 3.7 model is used by checking the logs in gdk tail -f gitlab-ai-gateway | grep "Request to LLM"

Screenshots

• Claude 3.7 model is used when feature flag is enabled:

• Claude 3.5 model is used when feature flag is disabled:

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #521400 (closed)