feat(init): add --checkout option to sam init command for specifying git branch, tag, or commit for cookiecutter projects

[MustafizSaadi]

Which issue(s) does this change fix?

#3555

Why is this change necessary?

Currently, sam init cannot clone a remote template and check out a specific branch/tag/commit in one step. The template have to be cloned first and then do a local checkout. Only then the specific branch/tag/commit could be utilized to initialize sam project which is not intuitive.

How does it address the issue?

The change adds a checkout flag with the sam init command. With the help of that the developers can checkout to their remote url custom branch, tag, or commit utilizing the sam init command. Also, added unit tests for checkout and updated the existing unit tests for the new checkout flag.

What side effects does this change have?

No side effects.

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.