Retrieve the version of the running Coniql

It would be useful to be able to query a running Coniql instance and get its version. This would probably simply be a new HTTP endpoint, returning simple plaintext information. It could also be appended to the Metrics endpoint, but I think that is not an ideal fit. It could also be a Query.

There's a security consideration when publishing such information - We don't know what sort of vulnerabilities may be found in our dependencies, and as such we want to be a little careful in publishing any information that could be used to identify attack surfaces. This is likely not all that significant though, as this repo is public and so therefore are our dependencies.