feat(manager): add GitManager

[angeloashmore]

Context

DT-1891

The Solution

Add GitManager (accessible via manager.git) with the following methods:

• createGitHubAuthState()
• fetchOwners()
• fetchRepos()
• fetchLinkedRepos()
• linkRepo()
• unlinkRepo()
• checkHasWriteAPIToken()
• updateWriteAPIToken()
• deleteWriteAPIToken()

The tests introduced in this PR cover ~80% of the code, which is sufficient for a first release.

Impact / Dependencies

This PR allows us to implement the Git integration UI.

This PR also introduces a drastically simpler method for writing manager tests (inspired by @xrutayisire's Playwright setup). Only the new GitManager tests use the new method, but we can migrate existing tests to the new method over time.

Checklist before requesting a review

• I hereby declare my code ready for review.
• If it is a critical feature, I have added tests.
• The CI is successful.
• If there could backward compatibility issues, it has been discussed and planned.

[linear]

DT-1891 AADev, I can use the manager to call SM API's Git endpoints

[vercel]

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Name	Status	Preview	Updated (UTC)
slice-machine	✅ Ready (Inspect)	Visit Preview	Feb 1, 2024 10:28pm

[bapmrl]

Excellent work! I really like the new createAPIFixture function.

[bapmrl]

I'm approving 😊 There's just a small conversation remaining. You can add a comment for the null value if you also think it's useful.