Athens: Create an HTTP endpoint that outputs the build version of the proxy

[arschles]

Is your feature request related to a problem? Please describe.

866 compiles the build date and version into the proxy binary. There's no way to get that from a running proxy, though

Describe the solution you'd like

We should expose the build info via an HTTP method.

Additional context

Follow-up from https://github.com/gomods/athens/pull/866#issuecomment-436134211

cc/ @marpio @robjloranger @manugupt1

[robjloranger]

This could possibly be part of the health endpoint, it just needs to return 200 for health check so could combine with some json or simple string info.

[marpio]

I would rather create an explicit endpoint for the version info, just to make it clear.
We could also return versions of the components we're using i.e. go, VCSs, storage.

[robjloranger]

I'll work on this. How about a /version endpoint that lists the server version for now and we can add more fields to the response body as we implement the availability of other data.

I'm thinking JSON response.