Extend environment settings to support prevent_self_review parameter

[brockgwex]

Prerequisites:

• Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.
  https://docs.github.com/en/actions/deployment/targeting-different-environments/using-environments-for-deployment#:~:text=prevent%20self%2Dreviews

• Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
  API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).
  https://docs.github.com/en/actions/deployment/targeting-different-environments/using-environments-for-deployment#:~:text=prevent%20self%2Dreviews

New Feature

Adds the ability to control whether prevent_self_review is enabled or disabled for a given deployment environment.

environments:
  - name: prod
    reviewers:
      - id: 5847163
        type: 'Team'
    prevent_self_review: true
    deployment_branch_policy:
      protected_branches: true