Convert input types based on MLflow's model signature

[adriangonz]

MLflow models provide a typing mechanism through their model signatures. These types are used to validate the input payloads, therefore is important that the mlserver-mlflow runtime takes them into account when converting from the V2 Protocol to an MLflow-valid input (see #160 and #161).

There is currently a mismatch between the types supported by the V2 Dataplane and MLflow native types. For example,

• MLflow allows you to set an input column as a string, whereas BYTES is the closest type in the V2 Dataplane.
• MLflow allows you to set an input column as binary, which is expected to be a base64 string. This is different to the BYTES field in the V2 Dataplane (which doesn't specify explicitly how these field needs to be transmitted).

This conversion will also need to take into account how to convert between the V2 "lower-level" types and MLflow's native types.