Done, commit 4476512292d900b91106da966bf8d12a7635cde4.

There's a prototype implementation of this (as AbstractElementValidationStage and StringElementValidationStage) in the ukf-mda project at commit 0846fd802641461b95062aa0b2131c04371e3f11:

https://github.com/ukf/ukf-mda/commit/0846fd802641461b95062aa0b2131c04371e3f11

This should be importable with the exception of the test case, which for local reasons uses a validator I don't think we want to upstream at this point (possibly never). It should be replaceable by a simple regex validator.

It might make sense to refactor the X509 certificate validator in terms of AbstractElementValidationStage, although that would introduce a requirement to configure an elementName property (although perhaps we can override the default?)