tests: Implement integration tests covering JumpStart PrivateHub workflows

[malav-shastri]

Issue #, if available:

Description of changes: Adding 6 new integ test cases for PrivateHub functionalities/workflows

Added tests: test_private_hub test_hub_model_reference test_jumpstart_hub_model test_jumpstart_hub_gated_model test_jumpstart_gated_model_inference_component_enabled test_instatiating_model

Testing done:

• all the newly added tests are passing
• integ tests
• unit tests
• black -l 100 .
• flake 8

Merge Checklist

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General

• [ ] I have read the CONTRIBUTING doc
• [ ] I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the Python SDK team
• [ ] I used the commit message format described in CONTRIBUTING
• [ ] I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
• [ ] I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• [ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
• [ ] I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes
• [ ] I have checked that my tests are not configured for a specific region or account (if appropriate)
• [ ] I have used unique_name_from_base to create resource names in integ tests (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[malav-shastri]

We should add tests for the fine-tuning/training paths as well.

I have purposely skipped it because training isn't supported for model references. And training for jumpstart model is already being covered in the test suit.