Implement model, model group and connector apis

[acha5066]

Description

This PR implements the model, model group and connector apis from the ML Commons API so that developers can set up connectors to 3rd party tools like OpenAI as described here: https://github.com/opensearch-project/ml-commons/blob/main/docs/remote_inference_blueprints/openai_connector_embedding_blueprint.md

Issues Resolved

Closes #169

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[acha5066]

• Added tests for the new namespace methods.
• Added sample usage to USER_GUIDE.md.
• Added documentation to the namespace class header. Linked to relevant API docs. Wasn't sure what documentation you had in mind so please let me know.

Regarding DCO I did signoff with Andrew Chappell and changed my username like: git config --global user.name "Andrew Chappell" so not sure why that isn't reflected. It keeps associating with andrew.chappell :thinking:

[codecov[bot]]

Codecov Report

Attention: 84 lines in your changes are missing coverage. Please review.

Comparison is base (9ebb183) 24.19% compared to head (e55cd04) 25.99%.

:exclamation: Current head e55cd04 differs from pull request most recent head 8a516e5. Consider uploading reports for the commit 8a516e5 to get more accurate results

Files	Patch %	Lines
...OpenSearch/Namespaces/MachineLearningNamespace.php	85.81%	20 Missing :warning:
...nts/MachineLearning/Connectors/DeleteConnector.php	50.00%	5 Missing :warning:
...points/MachineLearning/Connectors/GetConnector.php	50.00%	5 Missing :warning:
...s/MachineLearning/ModelGroups/DeleteModelGroup.php	50.00%	5 Missing :warning:
...s/MachineLearning/ModelGroups/UpdateModelGroup.php	50.00%	5 Missing :warning:
...h/Endpoints/MachineLearning/Models/DeleteModel.php	50.00%	5 Missing :warning:
...h/Endpoints/MachineLearning/Models/DeployModel.php	50.00%	5 Missing :warning:
...arch/Endpoints/MachineLearning/Models/GetModel.php	50.00%	5 Missing :warning:
...earch/Endpoints/MachineLearning/Models/Predict.php	50.00%	5 Missing :warning:
...Endpoints/MachineLearning/Models/UndeployModel.php	50.00%	5 Missing :warning:
... and 8 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #170 +/- ## ============================================ + Coverage 24.19% 25.99% +1.79% - Complexity 1938 2018 +80 ============================================ Files 246 263 +17 Lines 6865 7145 +280 ============================================ + Hits 1661 1857 +196 - Misses 5204 5288 +84 ```

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[acha5066]

DCO should be fine now

[dblock]

@shyim Got some time to CR this?