Enhance spanner migration script

Add sub_name translation. Populate missing data with safe defaults.
Solve issues around jsonb fields populate timestamp with rfc3339 format.

Using this script, I was able to view anomalies on the /a page on a
local instance and view them on graphs. Out of 68k v8 anomalies, only 3
don't have a match (because of empty spaces in test names).

Change-Id: If9bb88f6c54a27529a2e740be0f7fe3d7cc9989d
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/7793244
Reviewed-by: Eduardo Yap <eduardoyap@google.com>
Commit-Queue: Marcin Mordecki <mordeckimarcin@google.com>