Draft: POC repo embeddings

What does this MR do and why?

A simple POC which creates embeddings for a repo and then finds related files (epic &14107)

Use rails console to use it:

# generate embeddings for files in repo
CodeSuggestions::RepoEmbeddings.new(User.first).create_embeddings('/opt/gitlab-development-kit/ai-assist')

# find related files for a user's question
CodeSuggestions::RepoEmbeddings.new(User.first).related_for_questions

# find files related to another file
CodeSuggestions::RepoEmbeddings.new(User.first).related_for_files

Embeddings search seem to be promising for user's instructions (below for each question is nested list of top 5 related files for AI gateway project - https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist):

v2 endpoint for chat which accepts additional context
  ai_gateway/api/v1/chat/typing.py
  ai_gateway/api/v2/__init__.py
  ai_gateway/api/v1/chat/agent.py
  ai_gateway/api/v2/chat/agent.py
  ai_gateway/models/v2/anthropic_claude.py

check that JWT was issued by AI gateway
  ai_gateway/self_signed_jwt/token_authority.py
  docs/auth.md
  ai_gateway/auth/providers.py
  ai_gateway/self_signed_jwt/__init__.py
  scripts/tests/e2e_code_suggestions.py

authenticate user using JWT
  docs/auth.md
  ai_gateway/self_signed_jwt/token_authority.py
  ai_gateway/api/v1/code/user_access_token.py
  ai_gateway/self_signed_jwt/container.py
  ai_gateway/auth/providers.py

Where do we define completions API endpoint?
  ai_gateway/api/v2/__init__.py
  ai_gateway/api/v3/code/typing.py
  ai_gateway/api/v2/code/completions.py
  ai_gateway/api/v3/code/completions.py
  docs/api.md

Where do we authenticate user?
  docs/auth.md
  scripts/tests/e2e_code_suggestions.py
  ai_gateway/auth/user.py
  tests/code_suggestions/test_authentication.py
  conftest.py

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