feat: Add GitHub integration with agent_prompts and github_components

[julian-risch]

Related Issues

• Related to Add GitHub integration with agent_prompts and github_components #1650

• Related to Move Agent from experimental to main haystack-experimental#250
  @sjrl brought up that one way to keep example files from the experimental Agent around is an integration chore: Remove Agent after Haystack 2.12 release haystack-experimental#263 (comment)

Proposed Changes:

• Move github_components from experimental to a new integration
• Move agent_prompts from experimental to a new integration

The idea is to enable users to run the example notebook (or a version with updated imports) after having installed this new integration

How did you test it?

New unit tests and I ran all usage examples successfully with a test repo.

I haven't tested it with the notebook yet, which we would need to update first. (tracked by deepset-ai/haystack-cookbook#183 )

Notes for the reviewer

• I suggest we rename github_token parameter to api_key for consistency with many other integrations.
• Some components have github_token: Optional[Secret] = None, because they can work without any token while others use Secret.from_env_var("GITHUB_TOKEN"). I suggest we use Secret.from_env_var("GITHUB_TOKEN", strict=False) where we currently have None as the default.
• The internal implementation of the components differs in how they use _get_headers or _get_request_headers or define headers inline. We could refactor that.
• While we could find a way to set up integration tests, I would rather leave them out of this PR.
• GithubRepositoryViewer has a branch parameter in the run method, which could also be named ref to make more clear it can also be a tag or commit hash

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.