feature: Enabled update_endpoint through model_builder

[rsareddy0329]

Issue #, if available:
github issue: #2545
Description of changes:

• This code change enables update_endpoint through model_builder i.e., add update_endpoint to support session class to update endpoint instead of creating new one.
  Users will be able to use an optional flag(update_endpoint) in .deploy method accordingly based on preference to update existing endpoint or create a new one

Note:

• The change of Update endpoint is supported for single model endpoints, which updates existing endpoint with user provided model configuration instead of creating a new one.
• In case of multimodel, With the current code flow if endpoint already exists, a new model is added to the existing endpoint.

Testing done:

• Added relevant unit tests and verified this change of addition of update_endpoint flag.

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