Expand source support in ToolConfig

Bug: T402764

Currently, we have source_url support (fetching the ToolConfig from a http(s) url.

This change expands that feature to support fetching the ToolConfig from a git repository.

Example usage:

source:
  repository: https://gitlab.wikimedia.org/toolforge-repos/sample-complex-app-backend.git # Required
  path: deployment/toolforge.yaml # Required
  ref: main # Required

Relevant changes:

• source_url: https://example is deprecated in favour of source: { url: https://example }
• Warning is added in API response when source_url is specified
• git based logic is refactored into git.RemoteGitRepository
• tests.git added to test RemoteGitRepository, ref resolution is now tested
• Internal messages referencing source_url have been generified