Add ability to pass params through to action even if proto is defined

[hayesgm]

This patch adds the ability to pass params to action calls even when we've defined a proto. This is to simply the case when working with legacy code where data is included in parameters that are outside the scope of the defined protobufs. E.g. imagine this case:

curl http://example.com/ping?timeout=500

In this simple case, we would likely define the following route in HyperBuffs,

defmodule MyRouter do
  # ...

  get "/ping", HomeController, :ping, private: %{req: :none, resp: Defs.Pong}
end

defmodule HomeController do
  # ...
  def ping(conn, params) do
    Defs.Pong.new()
  end

That is, since there's no real body to parse, we accept nothing for the request protobuf. To accept the timeout field, we could either define a body:

message Ping {
  int32 timeout = 1;
}

defmodule HomeController do
  # ...
  def ping(conn, ping=%Defs.Ping{}) do
    :timer.kill_after(:timer.seconds(Ping.timeout))
    Defs.Pong.new()
  end

or, in this case, we could simply now call passthrough:

defmodule MyRouter do
  # ...

  get "/ping", HomeController, :ping, private: %{req: :none, resp: Defs.Pong, passthrough: true}
end

defmodule HomeController do
  # ...
  def ping(conn, params) do
    :timer.kill_after(:timer.seconds(params["timeout"]))
    Defs.Pong.new()
  end

The choice is whether or not the Ping message is truly a defined input schema. The thought is that legacy projects adopting HyperBuffs may allow some param input as they shift purely onto protobuf definitions.

Note: this can also be accomplished by calling conn.params["timeout"], but it does not seem it's the intention of Phoenix actions to dig into the conn object for parameters.