[FrameworkBundle] Added the possibility for the user to load any validation file wanted

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10936
| License       | MIT
| Doc PR        | ~

There is a question opened: is the fact that allowing any file finishing with ``validation`` or ``Validation`` ok?
I would not go for validation with a capital `V`.
IMO we should either look for `validation.yml` file in `Resources/config` directory or any validation files under `Resources/config/validation` directory as Doctrine does for ORM mapping.