refactor(dev-infra): improve type-safety of git client utility (#42468)

Currently the `GitClient` accepts a generic parameter for determining
whether the `githubToken` should be set or not. This worked fine so far
in terms of distinguishing between an authenticated and
non-authenticated git client instance, but if we intend to conditionally
show methods only for authenticated instances, the generic parameter
is not suitable.

This commit splits up the `GitClient` into two classes. One for
the base logic without any authorization, and a second class that
extends the base logic with authentication logic. i.e. the
`AuthenticatedGitClient`. This allows us to have specific methods only
for the authenticated instance. e.g.

  * `hasOauthScopes` has been moved to only exist for authenticated
    instances.
  * the GraphQL functionality within `gitClient.github` is not
    accessible for non-authenticated instances. GraphQL API requires
    authentication as per Github.

The initial motiviation for this was that we want to throw if
`hasOAuthScopes` is called without the Octokit instance having
a token configured. This should help avoiding issues as within
https://github.com/angular/angular/commit/3b434ed94d9ed067e5d999c064ae5f12b3cb175c
that prevented the caretaker process momentarily.

Additionally, the Git client has moved from `index.ts` to
`git-client.ts` for better discoverability in the codebase.

PR Close #42468

dev-infra/utils/github.ts

@@ -7,11 +7,12 @@
  */
 
 import {params, types} from 'typed-graphqlify';
+import {AuthenticatedGitClient} from './git/authenticated-git-client';
 
-import {GitClient} from './git/index';
 
 /** Get a PR from github  */
-export async function getPr<PrSchema>(prSchema: PrSchema, prNumber: number, git: GitClient<true>) {
+export async function getPr<PrSchema>(
+    prSchema: PrSchema, prNumber: number, git: AuthenticatedGitClient) {
   /** The owner and name of the repository */
   const {owner, name} = git.remoteConfig;
   /** The Graphql query object to get a the PR */
@@ -32,7 +33,7 @@ export async function getPr<PrSchema>(prSchema: PrSchema, prNumber: number, git:
 }
 
 /** Get all pending PRs from github  */
-export async function getPendingPrs<PrSchema>(prSchema: PrSchema, git: GitClient<true>) {
+export async function getPendingPrs<PrSchema>(prSchema: PrSchema, git: AuthenticatedGitClient) {
   /** The owner and name of the repository */
   const {owner, name} = git.remoteConfig;
   /** The Graphql query object to get a page of pending PRs */