fix: Handle SIGTERM process kill signal

[mildaniel]

Which issue(s) does this change fix?

#6688

Why is this change necessary?

Handles the case where a local service starts up, and is terminated by a non-control-c (SIGINT) signal.
For example, users running SAM CLI inside of a Docker container will see issues where containers aren't cleaned up since Docker sends a SIGTERM signal which is currently unhandled by SAM CLI.

How does it address the issue?

Adds a SIGTERM handler that unblocks the thread and control flow is handed back to the main thread to finish execution/clean-up.

What side effects does this change have?

N/A

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

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