sync: Log repo sync state events as 'data' events.

git_trace2_event_log.py:
+ Added LogDataConfigEvents method to log 'data' events.
  Sync's current_sync_state and previous_sync_state are logged
  as 'data' events in the current log.

  It logs are key/value in the |config| argument. Each key is
  prefixed with |prefix| argument.

  The following are sample events that are logged during repo sync.

   {"event":"data",
   "sid":"repo-20210914T181545Z-P000330c0/repo-20210914T181545Z-P000330c0",
   "thread":"MainThread",
   "time":"2021-09-14T18:16:19.935846Z",
   "key":"previous_sync_state/repo.syncstate.main.synctime",
   "value":"2021-09-14T17:27:11.573717Z"}

   {"event":"data",
   "sid":"repo-20210914T181545Z-P000330c0/repo-20210914T181545Z-P000330c0",
   "thread":"MainThread",
   "time":"2021-09-14T18:16:19.955546Z",
   "key":"current_sync_state/repo.syncstate.main.synctime",
   "value":"2021-09-14T18:16:19.935979Z"}

tests/test_git_trace2_event_log.py:
+ Added unit tests

sync.py:
+ Changed logging calls to LogDataConfigEvents.

Tested:
$ ./run_tests

Tested it by running the following command multiple times.
$ repo_dev sync -j 20
  repo sync has finished successfully

  Verified config data is looged in trace2 event logs.

Bug: [google internal] b/199758376
Change-Id: I75fd830e90c1811ec28510538c99a2632b104e85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/317823
Reviewed-by: Josh Steadmon <steadmon@google.com>
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

tests/fixtures/test.gitconfig

@@ -12,7 +12,7 @@
 	intm = 10m
 	intg = 10g
 [repo "syncstate.main"]
-	synctime = 2021-08-13T18:37:43.928600Z
+	synctime = 2021-09-14T17:23:43.537338Z
 	version = 1
 [repo "syncstate.sys"]
 	argv = ['/usr/bin/pytest-3']