feat: add support for assignees

[tdharris]

Description

Fixes #153

Add support for --assignees to add the provided GitHub users as assignees to the PR.

Note: According to Github - About issue and pull request assignees, we can leverage the Issues API since it is a "shared" action for both issues and pull requests: "Every pull request is an issue, but not every issue is a pull request."

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added --assignees flag to add the provided GitHub users as assignees to the generated Pull Requests.

Migration Guide

N/A.