Enceladus and Menas model compatibility

Currently we have no way of ensuring that the models used in Enceladus (std and conformance) are compatible versions with the ones stored in Menas. We can add a header to the HTTP requests that says what model version Enceladus is using, then Menas can check compatibility and respond accordingly.

If the header is not specified Menas should ignore the compatibility check to avoid breaking compatibility for people using Encealdus as a library.