feat: implement lazy loading for CLI groups

[bchampp]

Which issue(s) does this change fix?

Why is this change necessary?

The SAM CLI has many CLI groups now, and we see overhead while running a given command due to SAM needing to import all CLI sub-commands (and the modules they need).

How does it address the issue?

Implementing a lazy CLI group base class based on https://click.palletsprojects.com/en/stable/complex/#lazily-loading-subcommands. This defers imports so we only import the given CLI group when it's the command being run.

What side effects does this change have?

None - defer imports to be lazy loaded. I've added unit tests to validate the --help commands for these CLI groups, but if a requested CLI group doesn't exist (e.g. we're pointing to a non-existing group), the schema generation will fail.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• 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.

[reedham-aws]

Approving because I think it's good as is, just had one small suggestion

[roger-zhangg]

Do we have some before/after data comparison?

[bchampp]

Do we have some before/after data comparison?

Yeah - I was able to see about a ~300ms difference calling the --help CLI for a subcommand under local. I had to turn off the proxy module thing that gets added in development mode for it to be realistic since that also adds a lot of overhead.