🚀 [FEATURE] - Improve API for incoming request validation

### What you need?

Should be something like:

```
when! {
   req
   path => "/users/1",
   method => GET
   content_type => "plain/text"
   body ~= "^/(.*)/$"
}

then! {
   content_type => "application/json"
   body => b"{ \"name\" = \"Roger\" }"
}
```