Resolve "Add feature flag to introduce Claude 3.7 Sonnet"

What does this MR do and why?

Adds a feature flag code_generation_claude_3_7_sonnet to use Claude 3.7 Sonnet for code generation

Requires gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2034 (merged) to be merged before this can be merged

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Before	After

How to set up and validate locally

1. Feature.enable(:code_generation_claude_3_7_sonnet)

curl -X "POST" "http://gdk.test:3000/api/v4/code_suggestions/completions" \
     -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer AUTH_TOKEN' \
     -d $'{
  "project_path": "jashkenas/Underscore",
  "current_file": {
    "file_name": "user_test.js",
    "content_above_cursor": "// tests for createAssigner function",
    "content_below_cursor": ""
  },
  "instruction": "",
  "prompt_version": 2,
  "intent": "generation"
}'

Related to #521047 (closed)