Add endpoint for Github username reverse mapping

On production, we additionally have an endpoint that allows us to map Github usernames to the Eclipse user account. The current production endpoint has the same return as the account/profile endpoint with just a different way of looking up the user. An example of this endpoint on production is https://api.eclipse.org/github/profile/autumnfound

We will be setting this as an optional/stretch goal for 2023 Q1, and is an enhancement and isn't required for the success of the MBO.