Support generic actions in Form.for_action/3

[cschmatzler]

Is your feature request related to a problem? Please describe.
I'd like to have a generic way to create a form for all my resource actions. Right now, generic actions need to have their forms built manually since there's no generic Form.for_action/3.

Describe the solution you'd like
I'd like ash and ash_phoenix to support any action to be passed to Form.for_action/3, even if it's generic, When a generic action is passed, I should receive a form that has fields for all arguments of that generic actions. Obviously, there's no attributes to include.

Describe alternatives you've considered
It's not that difficult to generate a form myself with a schemaless Ecto changeset, but it's a different API instead of something more convenient.

Express the feature either with a change to resource syntax, or with a change to the resource interface

# in resource
 actions do
   action :foo do
     argument :bar, :string, allow_nil?: false

     # do things....
   end
 end

# in controller
  form = AshPhoenix.Form.for_action(Resource, :foo)
  # ^ this should return a form instead of raising a CaseClauseError
  # alternatively add a `for_generic_action/3` but I'd like the ergonomics of that less