fix(event_parsing): parse user access request system hook events

ParseSystemhook returned unexpected system hook type for four system hook events that GitLab documents:

• user_access_request_to_group
• user_access_request_revoked_for_group
• user_access_request_to_project
• user_access_request_revoked_for_project

These events are listed in the system hooks docs. Anyone receiving system hooks from GitLab got a parse error when a user requested access to a group or project, or cancelled that request.