Endpoint: supported versions

[keunes]

From #10 (comment)

We'll … need an endpoint about supported versions (e.g. a json array that contains all supported versions). matrix does this after the discovery, it has an endpoint https://$host/$prefix/versions. This might be our way-to-go as well, since this way it wouldn't even be necessary to put these info inside the .well-known file. That way we could take care of that endpoint at a later time.

[georgkrause]

I think a versions endpoint is cheap to implement, we should go this path. We only need to decide on a version format, I guess.

@Sporiff @JonOfUs How about v1 v2 ..., only allowing integers