Version endpoint

[morguldir]

Is your feature request related to a problem? Please describe.
Knowing the version of MAS that someone is running could be helpful for debugging and detecting MAS, (e.g. how many matrix-authentication-service instances exist in the wild)

Describe the solution you'd like
A version endpoint (e.g. _matrix/matrix-authentication-service/version for integrated mas), or /api/version -> matrix-authentication-service x.y.z (commit)

Describe alternatives you've considered
Usually you can ask if the person is using MAS, and which version they are using, but often it's quicker if you could just check yourself

Additional context
As an example the sliding sync proxy could be detected by sending a request to the msc3575 endpoint, and then you would see if they had their proxy set up, but it would be easier if there was a dedicated endpoint for it