The merge script currently accepts a configuration function that will
be invoked _only_ when the `ng-dev merge` command is executed. This
has been done that way because the merge tooling usually relies on
external requests to Git or NPM for constructing the branch configurations.
We do not want to perform these slow external queries on any `ng-dev` command
though, so this became a lazily invoked function.
This commit adds support for these configuration functions to run
asynchronously (by returning a Promise that will be awaited), so that
requests could also be made to the Github API. This is benefical as it
could avoid dependence on the local Git state and the HTTP requests
are more powerful/faster.
Additionally, in order to be able to perform Github API requests
with an authenticated instance, the merge tool will pass through
a `GithubClient` instance that uses the specified `--github-token`
(or from the environment). This ensures that all API requests use
the same `GithubClient` instance and can be authenticated (mitigating
potential rate limits).
PR Close#38223
Paul Gschwendtner