WIP: combined JSON API generator DSL

[Rylan12]

This is a WIP based on #21092.

This is just a temporary PR, because I haven't had a chance yet to resolve conflicts on the other PR.

Essentially, this adds a new, simple way to define the combined, reduced JSON schema based on the regular formula JSON using things like this:

class FormulaHash < T::Struct
  include Homebrew::API::GeneratorMixin

  # By default, this populates from the "name" key
  elem :name, String

  # Use `from` to specify which key to populate from. In this case, `hash["versions"]["stable"]`
  elem :stable_version, String, from: ["versions", "stable"]

  # Use nested structs
  elem :head_url, hash_as: HeadUrlHash

  # For more complex transformations, use a block on the original hash
  elem :complex, Integer do |hash|
    hash["some_key"].count
  end
end