feat: Support Github App tokens for Github backend

[soceanainn]

This change introduces support for Github App tokens. Supporting Github App tokens will allow for custom authentication through e.g. AWS Lambda.

There are essentially two differences when dealing with app tokens:

1. When fetching the repo and checking $.permissions.push in response (hasWriteAccess function), this value will never be true for app tokens (even those with appropriate permissions).
2. We can't fetch a user using the Github user endpoint for those tokens

For (1) we simply bypass the additional permissions check (validating that the repo can be fetched by the app token is probably enough for most cases).

For (2) we need to create a mock user object for use by StaticCMS (discussed below).

This change also aims to eliminate any redundant calls to fetch the user / repo from Github across the API and implementation code, by having at most one call each.

Creating a mock user for app token

Normally we fetch a GithubUser object using the Github API user endpoint and use that to represent the user (based on the token obtained through auth page).Github App tokens won't have a user defined for them so we cannot use the user endpoint.

Luckily we only seem to use three fields from this object in code: name, login and avatar_url. We can workaround this issue by setting defaults for those three fields by fetching the app, and using name and slug from response to set name and login respectively. avatar_url can be built from id in response.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 15.38462% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 55.79%. Comparing base (734cecd) to head (bbd1f99).

Files	Patch %	Lines
...ckages/core/src/backends/github/implementation.tsx	5.55%	15 Missing and 2 partials :warning:
packages/core/src/backends/github/API.ts	37.50%	4 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1100 +/- ## ========================================== - Coverage 55.83% 55.79% -0.04% ========================================== Files 259 259 Lines 12362 12378 +16 Branches 3110 3116 +6 ========================================== + Hits 6902 6906 +4 - Misses 5048 5058 +10 - Partials 412 414 +2 ```

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