Support pipeline templates manifests in custom Git locations

[kfr2]

Which issue(s) does this change fix?

#4107

Why is this change necessary?

This change will allow for multiple pipeline templates to be managed in one Git repository rather than needing to spread them across many repositories. This will greatly improve the developer experience of users who'd like to create pipelines from the templates as well as those in charge of maintaining them.

How does it address the issue?

When a custom Git location is passed to sam pipeline init, the utility will first look for a valid pipeline template manifest in the repository's root. If one is found, the user will be prompted to choose which pipeline to create. If one is not found, the repository will be treated as a standard pipeline template.

What side effects does this change have?

Although unlikely, this could potentially break automated systems which pass custom Git locations containing a manifest.yaml file.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

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

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