[Template Parameter Constraints] Custom validation

#377 Covers case where the constaints is valid with subtype relation

Need a way to add a custom validation for example: Checking that the type has a decorator used on it.
```cadl
@error
model Foo {}

model Bar {

}
```

## Things to fix when this is implemented
- [ ] Validate ResourceOperations argument has `@key` #375

## Proposal

https://gist.github.com/timotheeguerin/a9cde2423eb37cde14b1fad236066b23