Support for "validate" helper within rule blocks

Support for executing predicates using `validate` method within `rule` blocks will make it much easier to apply various checks with extra conditions:

```ruby
params do
optional(:per_page).filled(:integer)
end

rule(:per_page) do
if key?
key.failure("must be within 0..20 range") unless validate(gt?: 0, lteq?: 20)
end
end
```

See the original conversation: https://github.com/dry-rb/dry-validation/issues/678