Allow Validate macro to redefine it's output

[fnordfish]

I'm a big fan of using dry-validation contracts in TRB operations.
A cool, but too little advertised feature of a Dry::Validation::Result is it's output method, which returns only validated (and type-coerced) data. This makes it perfect for "forcing" a operation to only use safe data:

step Contract::Validate(name: :default, key: :customer)
step ->(options, params:, **) {
  params[:customer] = options["result.contract.default"].output
}

(Full Sample)

It would be great, if the Validate macro could take some option to get rid of the second step. Maybe something like the Nested macro:

step Contract::Validate(name: :default, key: :customer, output: ->(options, mutable_data:, key:, **) do
    { key => mutable_data["result.contract.default"].output }
end)

[apotonick]

For further reference, this should be filed here: https://github.com/trailblazer/trailblazer-macro-contract/issues

So basically, you want to (re-)assign an attribute with an arbitrary value. This is already possible in 2.1 by using an :output filter, but it's a cool idea to add this step automatically to Validate(). 🍻