Expose MLflow's model signature as metadata

[adriangonz]

MLflow models can define a model signature. The information contained in this model signature has a few parallelisms to the V2 Dataplane's model metadata. Therefore, it would be good to explore how we could convert from the model signature (which should be present in a MLflow's model artifact) to the model metadata "in-the-fly" so that the same information can still be exposed.

Note that there is currently a mismatch between MLflow's native types and the V2 Dataplane accepted data types (e.g. string vs BYTES). Therefore, there may be some loss of information when converting from the former to the latter format. We can explore further down the line how this loss of information can be minimised.