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[DRAFT] Generalize middleware options to per-class Data value objects

Demonstration / discussion PR. Right now every middleware that wants
typed accessors over its options has to hand-write the same boilerplate:

  DEFAULT_OPTIONS = { foo: nil, bar: nil, ... }.freeze
  attr_reader :foo, :bar, ...
  def initialize(app, **options)
    super
    @foo = @options[:foo]
    @bar = @options[:bar]
    ...
  end

Since `@options` was already frozen by design (Middleware::Base#initialize
post-PR #2696), the natural next step is to replace the Hash with a
per-subclass `Options = Data.define(...)` and let `Forwardable` cover the
accessor wiring.

Mechanism added in this draft:

- `Grape::Middleware::OptionsCompat` — a small mixin Options classes
  include to keep the legacy `options[:key]` idiom working (notably for
  `Middleware::Base#content_types` and `#content_type`). Unknown keys
  return `nil` to match Hash semantics.
- `Middleware::Base#initialize` detects `self.class::Options` and routes
  kwargs through `Options.new(**options)`. Subclasses that still rely on
  `DEFAULT_OPTIONS` Hash + deep_merge keep working unchanged.

Demonstrated on `Middleware::Formatter`:

- Replaces 5-line DEFAULT_OPTIONS Hash + 4-line `attr_reader` list +
  6-line initialize body with:

    Options = Data.define(:content_types, :default_format, :format,
                          :formatters, :parsers) do
      include Grape::Middleware::OptionsCompat
      def initialize(content_types: nil, default_format: :txt, format: nil,
                     formatters: nil, parsers: nil)
        super
      end
    end

    def_delegators :options, :default_format, :format, :formatters, :parsers

- Defaults move from the freeze'd Hash to `#initialize` signature.
- Immutability is implicit (Data instances).

Behaviour change: passing an unknown kwarg to a middleware whose `Options`
class doesn't declare it now raises `ArgumentError` instead of being
silently swallowed by `**options`. One form... (continued)

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