Expose source_merge_request_id and source_commit for resource state events via the API

Proposal

Currently there is no way for retrieveing the information on why an close event for an issue was generated. For example: when an issue is closed via an commit, the source_commit field is set to the commit sha that caused the close. However, this field is currently not exposed via the REST or GraphQL API, along with source_merge_request_id which also exists on the model ResourceStateEvent.

This makes 3rd party applications / API consumers impossible to display an "closed via commit" note in an issue or similar tasks.