Add `./mach timings`

[SimonSapin]

Summarizes the time taken by tasks in given task group IDs

Example:

$ ./mach timings DL4zftVfSqW3WOTC3IoFcg DBt9ki9gTdWmwAk-VDorzw HKX2tko_RgGd6--031IfOA
https://community-tc.services.mozilla.com/tasks/groups/DL4zftVfSqW3WOTC3IoFcg
count 1, total 0:00:29, max: 0:00:29    docker  0:00:29
count 2, total 1:21:41, max: 0:47:55    macos-disabled-mac8 0:47:55 0:33:45
count 6, total 2:47:04, max: 0:37:12    macos-disabled-mac8 WPT 0:23:30 0:29:24 0:28:13 0:37:12 0:20:35 0:28:09
https://community-tc.services.mozilla.com/tasks/groups/DBt9ki9gTdWmwAk-VDorzw
count 1, total 0:00:32, max: 0:00:32    docker  0:00:32
count 1, total 0:59:14, max: 0:59:14    macos-disabled-mac1 0:59:14
count 6, total 4:12:16, max: 1:01:14    macos-disabled-mac1 WPT 0:40:29 0:18:55 0:46:50 0:44:38 1:01:14 0:40:10
count 1, total 0:55:19, max: 0:55:19    macos-disabled-mac9 0:55:19
count 6, total 4:25:09, max: 1:01:40    macos-disabled-mac9 WPT 0:37:58 0:37:24 0:27:18 1:01:40 0:46:17 0:54:31
https://community-tc.services.mozilla.com/tasks/groups/HKX2tko_RgGd6--031IfOA
count 1, total 0:00:49, max: 0:00:49    docker  0:00:49
count 7, total 2:49:55, max: 0:32:28    macos-disabled-mac8 WPT 0:32:28 0:29:55 0:08:55 0:19:44 0:27:36 0:31:41 0:19:35
count 7, total 3:30:39, max: 0:45:07    macos-i3 WPT    0:23:23 0:37:31 0:31:03 0:37:58 0:45:07 0:08:17 0:27:20
count 13, total 4:39:15, max: 0:38:49   macos-i7 WPT    0:37:12 0:20:01 0:18:59 0:17:29 0:38:49 0:17:00 0:20:02 0:26:49 0:20:03 0:09:17 0:20:02 0:19:55 0:13:39