Add label option

[sercantor]

Description

Addresses #134.
GH api doesn't seem to have the option to provide labels with the PR request endpoint, that seems to be in their graphql api.

Added label flag, and a request to add the label after the PR is opened successfully (similar to requesting reviewers)

I have no idea how to write go, but I've added this since I looked at the code, and it looked similar to how you add reviewers to the pr, I've provided a description to why I'd like to use this in the issue I've opened, but if it doesn't suit the CLI's usecase I'll close this PR.

Oh, also I didn't really test it with code, I just made the changes, built it, ran it in my cli for some of my own repos, seems to work OK.

TODOs

Read the Gruntwork contribution guidelines.

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

Release Notes (draft)

Added the option to pass labels to the PRs