Update New-GitHubBranch pipeline support

[HowardWolosky]

Description

• Updates New-GitHubBranch to be able to take a GitHub.Branch object as pipeline input (for the base branch)
• Updates New-GitHubBranch to be able to take in a Sha so that a branch can be created from an arbitrary commit (which also enables a new branch to be created from a GitHub.Branch pipeline input value without needing to perform an additional query on BranchName to get its Sha)
• Updates GitHub.Branch to have Sha as a top-level property.
• Updated existing tests and added additional tests.

Issues Fixed

Fixes #261

References

GitHub Refs API

Checklist

• You actually ran the code that you just wrote, especially if you did just "one last quick change".
• Comment-based help added/updated, including examples.
• Static analysis is reporting back clean.
• New/changed code adheres to our coding guidelines.
• Formatters were created for any new types being added.
• New/changed code continues to support the pipeline.
• Changes to the manifest file follow the manifest guidance.
• Unit tests were added/updated and are all passing. See testing guidelines. This includes making sure that all pipeline input variations have been covered.
• Relevant usage examples have been added/updated in USAGE.md.
• If desired, ensure your name is added to our Contributors list

[HowardWolosky]

/azp run PowerShellForGitHub-CI

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).