This commit makes sure that the Spring `Validator` used by the MVC
layer doesn't expose a JSR-303 contract, if any.

The default implementation of the `mvcValidator` is
`LocalValidatorFactoryBean`. While this object is exposed as a Spring
`Validator` only, its runtime capabilities expose that contract as well
as the standard `Validator` and `ValidatorFactory` ones.

Concretely, if an auto-configuration is checking if a
`javax.validation.Validator` bean is missing, the condition will match
since we only know about "advertized types": beans haven't been created
yet so we can't inspect their runtime capabilities. Since the condition
match, we will auto-configure a bean. At runtime though, we're no longer
ale to inject a `javax.validation.Validator` by type since two candidates
are available.

This commit introduces `SpringValidatorAdapterWrapper`, a wrapper class
on any `SpringValidatorAdapter` (`LocalValidatorFactoryBean` being one of
the available implementations) that only exposes the Spring contract.

Also, if a `javax.validation.Validator` bean is available, we will use it
for the MVC layer, rather than creating a new one.

Closes gh-8223