[github] Add ability to override values on the GitHub client

### TL;DR

Users may need the ability to override github client values like API url, etc. This should be implemented and variables added for configuration by the end user.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_

Find the GitHub client implementation in the codebase, likely in a package like `internal/github` or `pkg/github`. Look for where the API URL and other client configurations are currently set. The change involves making these values configurable, possibly via environment variables or a config struct. Check for existing tests for the client to understand the pattern for modification.

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