Pydantification

[JGoldstone]

This PR converts camdkit to use Pydantic. As compared to the previous PR attempting to do the same:

• it is on a new branch off of main, and can be automatically merged.
• references to developer-system-local regression references have been converted to references inside the source tree; in particular, copies of the pre-Pydantic schema and subschemas and examples are now in ...test/resources/.

[jamesmosys]

I've looked through this and I think the new approach is excellent. @JGoldstone can you confirm that the outputs - i.e. build/schema.json and the build/examples/* are identical to the published ones?

[JGoldstone]

Will check the match one more time. Almost 100% sure the examples match; will recheck schema, but I don't think I would have said it's ready without that matching as well.

[JGoldstone]

The example regressions were running but because of a missing main() in test_schema_regression.py, that coefficient wasn't running. Are you OK with these differences? If something isn't self-evident I can go over the rationale item-by-item:

[JGoldstone]

To go through those verbally (and they all have to do with the new-ish PTP stuff):

• Pydantic writes a real number having an inclusive lower bound of zero as 0.0 as opposed to the hand-coded 0
• The description was missing in the hand-coded schema for PTP synchronization priorities so I added it
• There were no bounds on the leader identity string: it could be zero-length, it could be 4 GB
• There was no upper bound on the vlan integer. I was able to find instances on the web where it was greater than 65535, so I made it have a max of an unsigned 32-bit int
• There was no validity check on the values of the time source string, so I looked into the code and found the three values we define and limited acceptable values to one of "GNSS", "Atomic clock" and "NTP"

[JGoldstone]

If you're good with those, I will hand-update the reference schema json against which the generated schema is compared to be as shown above, then check in that and the fix to add main() etc to test_regression_schema.py. And push that change.

[jamesmosys]

The minor schema changes LGTM.