get_files() and file tree support

[maciekbanas]

Part of #754.

Implement file-related functionality for BitBucket Cloud:

• File tree (get_repos_trees()): Already partially implemented via GET /2.0/repositories/{workspace}/{repo_slug}/src/{branch}/ with recursive listing. Verify and add tests.
• File content (get_files()): Retrieve file content via GET /2.0/repositories/{workspace}/{repo_slug}/src/{branch}/{path}. Map to the standard gitstats_files table.
• Code search (with_code parameter): BitBucket Cloud has GET /2.0/search/code — implement search_for_code() and search_repos_for_code() in EngineRestBitBucket.
• Unit tests with mocked API responses.