Extensible validation support in Volto forms

[sneridagh]

Supersedes #6161
Refactored and re-document for clarity and simplicity from it.

@stevepiercy I think now it's the good one. I'm more satisfied with this one now. I will review myself again, but I think it's ready for check.

@davisagli I've refactored it a bit after talking to @robgietema in lieu of clarity and simplicity. Also reflected it in the docs too. Decisions I made, we can talk about it:

Create an API for registerUtility

I think this is much more cleaner and fixes the thing you've mentioned in the other PR, for the "trying to push the square shaped piece in the circle shape hole". Also, I took it from a previous conversation last week with @ericof but that's another story. TL;DR: it was about the convenience of having an API that does and registers things for you, whatever it is, if it's a widget or a view, or other thing where you should "touch" and manually set things in different places. This makes much more sense to me now, with the current shape.

It will also make sense in other scenarios where I think we will need such "named utilities", eg. in other frameworks, having utility helpers, like flattenToAppUrl, globally, then ask for them in a named way.

Using the format property from JSON schema specs

@robgietema pointed me out to this one, I'm using the default property for defining named validators as defined in the spec.

I still have to cleanup, will continue working polishing this a bit.

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📚 Documentation preview 📚: https://volto--6181.org.readthedocs.build/

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[sneridagh]

@davisagli @stevepiercy I think I went through all the suggestions, let me know if I left something!

[tiberiuichim]

Just to continue on my though on on-offs from the previous PR: I could implement a generic validator called customFunction, which reads the field definition from schema, finds a validate function in the schema and uses that for actual implementation. So I can have my "one-off" validation helper closer to the schema.

[sneridagh]

@tiberiuichim yeah, I think you could do that already with the current implementation. However, that would only work with not serializable blocks defined in code.

[stevepiercy]

Looks pretty good. I am uncertain about one of my suggestions for the code sample.

[sneridagh]

@tiberiuichim I implemented your blocksErrors prop suggestion.

[ichim-david]

@sneridagh my only concern left is regarding the type validator as we've spoken if field shouldn't also be suffixed with ? to mark it as optional as both isNumber and startEventDateRangeValidator makes no direct use of the field
https://github.com/plone/volto/pull/6181/files#diff-0fb5051092a06c331a0607394901c0217dc85e7d2f3ce393fadab348e0832b56R298

[sneridagh]

@ichim-david done.

[sneridagh]

Merging! Thanks to all for the effort and for having this done!