Support for pattern-matched rules

In order to make defining rules that depend on additional conditions nicer, we can have something like this:

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

rule(:per_page).if(:key?).validate(gt?: 0, lteq?: 20)
```

See the original conversation: https://github.com/dry-rb/dry-validation/issues/540#issuecomment-744022950