[DRAFT] Add Model Registry to Unified SDK

[syntaxsdev]

What this PR does / why we need it:

Adds Model Registry to KF SDK (as a baseline) by importable extra install.

This is inline with issue #59

Use case:

from kubeflow.trainer import TrainerClient
from kubeflow.model_registry import ModelRegistryClient

We also have original ModelRegistry aliased
This immediately allows users to use surfaced ModelRegistryClient

from kubeflow.model_registry import ModelRegistry

We do not have the mr_openapi added yet because it is not surfaced via MR client, so a separate fix will come later once surfaced into MR, but it is not super urgent since mr_openapi was never surfaced in the first place in the model registry client.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #59

Checklist:

• Docs included if any changes are user facing

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[kramaranya]

Thank you @syntaxsdev! Before we proceed with the implementation, I think we should first discuss which approach would be most efficient and maintainable long-term in the original issue #59