sync: support post-sync hook in <repo-hooks>

Add support for a new hook type "post-sync" declared in the manifest using
<repo-hooks>. This allows executing a script automatically after a successful
`repo sync`.

This is useful for initializing developer environments, installing project-wide
Git hooks, generating configs, and other post-sync automation tasks.

Example manifest usage:

  <project name="myorg/repo-hooks" path="hooks" revision="main" />
  <repo-hooks in-project="myorg/repo-hooks" enabled-list="post-sync">
    <hook name="post-sync" />
  </repo-hooks>

The hook script must be named `post-sync.py` and located at the root of the
hook project.

The post-sync hook does not block `repo sync`; if the script fails, the sync
still completes successfully with a warning.

Test: Added `post-sync.py` in hook project and verified it runs after `repo sync`

Bug: b/421694721
Change-Id: I69f3158f0fc319d73a85028d6e90fea02c1dc8c8
Signed-off-by: Kenny Cheng <chao.shun.cheng.tw@gmail.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/480581
Reviewed-by: Scott Lee <ddoman@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

hooks.py

@@ -25,6 +25,7 @@ from git_refs import HEAD
 # The API we've documented to hook authors.  Keep in sync with repo-hooks.md.
 _API_ARGS = {
     "pre-upload": {"project_list", "worktree_list"},
+    "post-sync": {"repo_topdir"},
 }