feat: Add sourcery assistant review workflow

[brendanator]

In particular there are a couple of runs that check permissions work allow the workflow to succeed:

• Default permissions - job fails with permissions error
• Contents and pull-request permissions - job succeeds

This is even with restricted GITHUB_TOKEN permissions:

[github-actions]

PR Type: Enhancement

Summary of PR: This PR adds a GitHub Actions workflow to trigger Sourcery Coding Assistant reviews on pull requests when a review is requested or a specific label is added.

General PR suggestions

• Ensure that the GitHub secret SOURCERY_TOKEN is properly set up in the repository settings.
• Verify that the sourcery-nightly package is the desired choice over the stable version for the project's needs.

[github-actions]

PR Type: Enhancement

Summary of PR: This PR adds a GitHub Actions workflow to trigger Sourcery Coding Assistant reviews on pull requests when a review is requested or a specific label is added.

General PR suggestions

• Ensure that the Sourcery token is securely stored and has the correct permissions.
• Verify that the triggers for the workflow align with the team's development workflow and practices.
• Consider specifying a version for the sourcery-nightly package to avoid potential issues with breaking changes in future nightly releases.

[github-actions]

PR Type: Enhancement

Summary of PR: This PR adds a GitHub Actions workflow to trigger Sourcery Coding Assistant reviews on pull requests when a review is requested or the "sourcery-review" label is added.

General PR suggestions

• Ensure that the GitHub secret SOURCERY_TOKEN is properly set up in the repository settings.
• Verify that the sourcery-nightly package is the desired choice over the stable version for the project's needs.