Access params when coercing

Encountered this use-case today and feel like it would make for a nice enhancement to the coercion functionality to be able to access the params from my coercion method.

I have an API endpoint which accepts an address (street, city, state, zip) however the city and state parameters are optional. In most countries you can derive both from the zip/postal code, so I'd like to be able to do something like:

params do
  requires :street, type: String
  requires :zip, type: String
  optional :city, type: String, coerce_with: ->(val) { val ? val : LookupService.find_city(params[:zip]) }
  optional :state, type: String, coerce_with: ->(val) { val ? val : LookupService.find_state(params[:zip]) }
end

Ultimately ended up doing this with a callback, updating the params hash.

Start by tracing Grape's coercion functionality and the callback-based workaround described in the issue. Determine how a coercion method could access the surrounding params, including optional city and state values, and define the expected behavior for dependent fields. Done means the supported behavior is covered by tests for the postal-code lookup example.