Allow customer to use Claude Haiku model for code completion

What does this MR do and why?

Adds new feature flag use_claude_code_completion for a customer who needs to use Anthropic for code completion as code-gecko is being deprecated and onboarding a new model for code completion will be a long process. They already have the Claude family of models approved.

See https://gitlab.com/gitlab-org/gitlab/-/issues/523902 for more details

Requires AIGW MR gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2137 (merged)

How to set up and validate locally

1. Feature.enable(:use_claude_code_completion, Group.find(YOUR_USER.duo_available_namespace_ids.first))

curl -X "POST" "http://gdk.test:3000/api/v4/code_suggestions/completions" \
     -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer AUTH_TOKEN' \
     -d $'{
  "current_file": {
    "file_name": "test.go",
    "content_above_cursor": "// write a function that parses a .env file",
    "content_below_cursor": "//end function"
  },
  "prompt_version": 2,
  "intent": "completion"
}'

model.name: "claude-3-5-haiku-20241022" in response.

curl -X "POST" "http://gdk.test:3000/api/v4/code_suggestions/direct_access" \
     -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer AUTH_TOKEN' \
     -d $'{}'

Should return "message": "403 Forbidden - Direct connections are disabled"

MR acceptance checklist

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Related to #524897

Related to #523902