Add timing keyword argument for hooks

Measure the duration of the sync operation in the Execute method of the
Sync command and pass it to post-sync hooks as a standard keyword
argument (`sync_duration_seconds`).

Updates based on code review:
- Update _API_ARGS in hooks.py to allow sync_duration_seconds for post-sync hooks.
- Do not cast sync_duration_seconds to int for better granularity.
- Update docs/repo-hooks.md to document sync_duration_seconds.
- Add unit test for argument validation in test_hooks.py.

Test: Ran run_tests using venv python, all 554 tests passed.
Bug: TBD
Change-Id: Ie29e002a5d283460d993ad96c224dbf4b6d7985c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/575021
Tested-by: Arif Kasim <arifkasim@google.com>
Commit-Queue: Ram Peri <ramperi@google.com>
Reviewed-by: Arif Kasim <arifkasim@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

hooks.py

@@ -25,7 +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"},
+    "post-sync": {"repo_topdir", "sync_duration_seconds"},
 }