Add option to use Github Enterprise API for api key checks

Augur is difficult to use for people using github's [enterprise api](https://docs.github.com/en/enterprise-cloud@latest/rest/issues/issues?apiVersion=2022-11-28) (`api.SUBDOMAIN.ghe.com`) because keyman seems to validate credentials with a [hardcoded API path](https://github.com/chaoss/augur/blob/9050232e/augur/tasks/github/util/github_api_key_handler.py#L12).