Add support for SAGEMAKER_MODEL_SERVER_TIMEOUT_SECONDS variable

Issue #, if available:

Description of changes: This provides a way for the customer to provide a timeout value specifically in seconds. This is needed due to a bug in multi-model-server that interpreted the existing timeout value in minutes even though it was documented to be in seconds.

We introduced a new variable rather than changing the meaning of the old variable out of concerns around backward-compatibility. We can revisit this later if needed.

Testing done:

Unit tests all passed.

Installed modified versions of MMS and this toolkit in a container and ran an end-to-end example to trigger the timeout to verify it works. It does :)

Merge Checklist

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General

[ ] I have read the CONTRIBUTING doc
[ ] I used the commit message format described in CONTRIBUTING
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[ ] I have updated any necessary documentation, including READMEs

Tests

[ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
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aws / sagemaker-inference-toolkit