Add support for repository-level immutable releases settings API

[sheeeng]

Now that organization-level immutable releases settings are supported (via #3771 / #3774), it would be great to also add support for the repository-level immutable releases settings API.

The GitHub REST API provides three endpoints for managing immutable releases at the repository level:

• Check if immutable releases are enabled for a repository — GET /repos/{owner}/{repo}/immutable-releases
• Enable immutable releases for a repository — PUT /repos/{owner}/{repo}/immutable-releases
• Disable immutable releases for a repository — DELETE /repos/{owner}/{repo}/immutable-releases