Move POST /mlflow/model-versions/create endpoint.

[dsuhinin]

There is a problem to fully finish current PR. Problem related to original mlflow implementation which works under the good and how tests organized. The main problem, that the mlflow Python tests expect to have version property in 2 different types: string type in RESTFul tests and int type in store tests.

Current go implementation is using the single entry point for both RESTful and store parts and using the proto objects to receive and return information. As a result some of Python tests just failing, because version property is always in string format which is not fully correct.

The possible and maybe the most correct solution is to modify original mflow logic to have version property in the same type for both RESTful and store parts. It could be a problematic, because it needs to change a proto contract or internal logic.