[Validator] Add Validation::createCallable()

[W0rma]

I just took the examples from https://symfony.com/blog/new-in-symfony-5-1-validator-improvements#validation-callables and #15093

Fixes #13045
Fixes #15093

[carsonbot]

Hey!

Oh no, it looks like you have made this PR towards a branch that is not maintained anymore. :/
Could you update the PR base branch to target one of these branches instead? 4.4, 5.3, 5.4, 6.0.

Cheers!

Carsonbot

[W0rma]

@carsonbot Thank you for the hint. I rebased against 5.3

[wouterj]

Hi @W0rma! This is a great contribution, this feature was missing docs for quite some time.

I have some suggestions on the location/content of this article. Please let me know if you don't agree or if you don't have the time to make the changes, I'm sure we can get this merged in any case :)

• We're trying to reduce the number of small "sub articles" and move things into the main article. What do you think about moving this contribution to validation.rst, as a new section under "The Basics of Validation"?
• I think we can be a bit more to-the-point in the main validation article. E.g. something like:

  The ``Validation`` also allows you to create a closure to validate values
  against a set of constraints (e.g. in ... or ...):
  
  ``createCallable()``
     This returns a closure that throws ... when the constraints aren't matched.
  ``createValidCallable()``
     ...

• Then, we can advocate how nicely this plays with other components in their docs. E.g. add a little tip with your examples to the options resolver and console question helper docs.

[W0rma]

@wouterj Thank you for the suggestions. I think they are all very good ideas and therefore I applied them to this PR.

[javiereguiluz]

@W0rma great contribution! Thanks a lot!

While merging we added some references to better link sections between them. See 4405199.

[ngrie]

@W0rma @javiereguiluz I just stumbled over this in the docs and noticed that the mentioned method createValidCallable() does not exist, probably the createIsValidCallable() method should be referenced instead. Or am I missing something?

[javiereguiluz]

@ngrie good catch! We've just fixed that in 408fd80 and 4b01042