[DRAFT] Support CustomAttributes / request context in handler function overrides

Issue #, if available: #110

Description of changes:

Initial draft of a non-breaking option to support passing an extra "request context" parameter to handler override functions (transform_fn, input_fn, predict_fn, output_fn), ONLY when the signature of the provided function includes the extra parameter.

This should allow script mode users to access additional request context (such as the SageMaker CustomAttributes header) without breaking implementations using the existing function APIs.

Outstanding TODOs (hoping for input from reviewers!) include:

[ ] Agree the scope of the additional "context" object - the raw context object as per Transformer.transform() seems maybe too broad & not very user-friendly? I'm not even sure what API it offers
[ ] Validate the approach
[ ] Understand & complete relevant doc updates & downstream library updates (e.g. add the parameter in default handler fns here and in other libraries like the PyTorch and HuggingFace toolkits too? Update SageMaker Python SDK docs when new DLCs are available with this change built in?)

Testing done:

Added some unit tests to show initial non-breaking-ness, but would be nice to get an integration test too maybe?

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aws / sagemaker-inference-toolkit