Add new plugin type for custom schema validators

[eslavich]

This resurrects #1050 so that we can merge it to master and include in a 2.x release.

[CagtayFabry]

I will try to run this against the previous test branch again soon @eslavich

[WilliamJamieson]

@eslavich is it possible to break this into smaller chunks?

[braingram]

Overall this looks great.

I made a few minor documentation suggestions, changed some imports to relative and highlighted one test coverage gap (because of the change in ndarray validator scope).

Edited: removed the relative imports per Williams objection

[eslavich]

@braingram I think I've addressed all of your comments. The weldx failures are due to them using the old-style validators in their new-style extension:

https://github.com/BAMWelDX/weldx/blob/master/weldx/asdf/extension.py#L62-L66

As for stdatamodels, the same tests fail on asdf master so I don't think they're related to the changes in this PR:

https://github.com/asdf-format/asdf/actions/runs/4099643240/jobs/7069745475

[CagtayFabry]

@braingram I think I've addressed all of your comments. The weldx failures are due to them using the old-style validators in their new-style extension:

https://github.com/BAMWelDX/weldx/blob/master/weldx/asdf/extension.py#L62-L66

Would it help to remove the code you linked from the new style extension to see if the legacy implementation still works @eslavich ?

[CagtayFabry]

I have removed the incompatible validators from the new style extension, can you run the weldx downstream test again @eslavich ?

[eslavich]

I have removed the incompatible validators from the new style extension, can you run the weldx downstream test again

Thank you! It looks like someone re-ran the test for me and it passed this time.

[braingram]

@eslavich is it possible to break this into smaller chunks?

@WilliamJamieson do you have a proposal for breaking this up or are you happy with it as is?

If you're happy with it I will rebase and merge this PR.