feat(scm): Add vendor-agnostic interfaces

[cmanallen]

Add actions and tests (prune unnecessary; add missing):

• get_issue_comments
• create_issue_comment
• delete_issue_comment
• get_pull_request_comments
• create_pull_request_comment
• delete_pull_request_comment
• get_comment_reactions
• create_comment_reaction
• delete_comment_reaction
• get_issue_reactions
• create_issue_reaction
• delete_issue_reaction

Each action has its own contract. It needs to return a generic response that effectively communicates what callers want to receive. It should also include the raw response and the provider type so that deeper introspection can be accomplish for service specific customization.

Add provider implementations:

• GitHub
• GitLab (depending on how fast the GitLab project progresses -- do last)

Integration tests for:

• is_rate_limited
• is_rate_limited_with_allocation_policy
• map_integration_to_provider
• map_repository_model_to_repository
• fetch_service_provider
• fetch_repository

Integration tests for:

• GitHub provider
• GitLab provider

[GabeVillalobos]

Instead of using the client directly, would it make sense to add to the existing GitHubIntegration class? While that class is fairly large in implementation, it does include quite a bit of logic that could simplify some of these provider implementations.

The existing class also includes feature-based metric collection and error handling that would need to be reimplemented here as well.

[cmanallen]

We'll work our way up to it if necessary. Likely we'll just port features from that class as we'll need a generic contract design and callers will need to conform. Leaving existing code in place means the migration path is async.