Allow users to enable authentication for rest proxy

[anishasthana]

We currently allow users to enable a rest proxy for models -- it would be neat if we support authentication via something like oauth proxy so that access to models can be restricted if desired.

We're prototyping out a solution in ODH that adds an oauth proxy container, but the implementation currently is a little hacky and relies on annotations. Ideally we could add a flag under https://github.com/kserve/modelmesh-serving/blob/main/config/default/config-defaults.yaml#L27 that defaults to "false" which would then inform whether servingruntime pods come up with an oauth proxy container.