KCIDB: v5.1 - origin's build/test finish timestamps

Hello again, everyone involved with sending data to KCIDB,

We've mostly finished implementing support for v5.0, so it's time for another
update. This time backwards-compatible.

We would like to add two fields to checkouts: "origin_builds_finish_time" and
"origin_tests_finish_time". They're supposed to contain the time the CI system
which sent the checkout has finished running the builds/tests it planned for it.

These fields are requested by the KernelCI Webdashboard team to help implement
marking a CI system's checkout as "done building" and "done testing". Note
that each revision can have multiple checkouts, from one, or more CI systems.

The new fields duplicate the general (but harder to implement) mechanism
supported thus far: sending individual planned builds/tests without their
status, and then sending their status when they're done.

Here's the corresponding PR:

https://github.com/kernelci/kcidb-io/pull/97

Please comment there, or in response to this message.

As the change is minor and backwards-compatible, I'd like to set the feedback
deadline to Wed, Mar 19. If there are no objections by that time, I'll merge
this change to the schema and database support on that day.

Nick