Allow inference runtimes to define metadata defaults

[adriangonz]

trafficstars

There are a few fields in the model metadata object that could make sense to let runtimes define defaults for. For example, the SKLearnRuntime could set the platform value to mlserver-sklearn/<version>.

This could also be valuable for the content type encodings, so that runtimes could e.g. set the default codec to be used to np (or pd, for the MLflow runtime).