[Validator] Add ClassMetadata plural methods for convinience

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #4143
| License       | MIT
| Doc PR        | -

I realised there's no specific place to document this methods, as the code examples always include all the formats. I think it's enough if IDE autocompletes these methods.