API Coverage: Add Support for Deployment Environments

[X-Guardian]

Description

This PR adds the following new cmdlets for managing GitHub Deployment Environments:

• New-GitHubDeploymentEnvironment
• Get-GitHubDeploymentEnvironment
• Set-GitHubDeploymentEnvironment
• Remove-GitHubDeploymentEnvironment

Issues Fixed

• Fixes #394

References

• Deployment Environments

Checklist

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

[X-Guardian]

@HowardWolosky, this PR is ready for review.

[X-Guardian]

@HowardWolosky, any timescale on when you will be able to re-review this PR?

[X-Guardian]

@HowardWolosky, any update on the review for this PR?

[HowardWolosky]

/azp run PowerShellForGitHub-CI

[azure-pipelines[bot]]

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