Avoid 'last arg as keyword param' warning when building user middleware

This change marks the methods which construct user middleware, so they are
called with the 2.6 keyword argument behaviour in 2.7 (and avoid printing a
warning about it).

Some users add their own middleware & write their own constructors with named
parameters, and use keyword arguments when adding their middleware to the
builder:

```
class MyMiddleware < Faraday::Middleware
  def initialize(app, my_argument:)
    @app = app
    @my_argument = my_argument
  end

[...]

connection = Faraday.new() do |configuration|
  configuration.use MyMiddleware, my_argument: 'hello'
end
```

This works great in 2.6 & 2.7, but 2.7 prints a deprecation warning:
```
/Users/danielholz/.gem/ruby/2.7.0/gems/faraday-1.0.1/lib/faraday/dependency_loader.rb:21: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/danielholz/code/some_project/lib/some_project/faraday_middleware.rb:2: warning: The called method `initialize' is defined here
```
The warning is a bit frustrating, since the calling code is not in the user's
code, so they cannot add `**` to the call. The best option for them is to
change their middleware to accept a Hash as a positional parameter & pass a
Hash when adding their middleware (and checking for required parameters
themselves).

I tried an alternative to `ruby2_keywords` by adding an explicit `**kwargs`
argument to `Faraday::RackBuilder#use`, `Faraday::RackBuilder#use_symbol`,
`Faraday::RackBuilder::Handler#initialize`, and
`Faraday::DependencyLoader#new`, but it hit the explicit delegation of keyword
arguments issue
(https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/#why-deprecated)
in 2.6.

lib/faraday/dependency_loader.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+def ruby2_keywords(*) end if RUBY_VERSION < '2.7'
+
 module Faraday
   # DependencyLoader helps Faraday adapters and middleware load dependencies.
   module DependencyLoader
@@ -13,7 +15,7 @@ def dependency(lib = nil)
       self.load_error = e
     end
 
-    def new(*)
+    ruby2_keywords def new(*)
       unless loaded?
         raise "missing dependency for #{self}: #{load_error.message}"
       end

lib/faraday/rack_builder.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+def ruby2_keywords(*) end if RUBY_VERSION < '2.7'
+
 require 'faraday/adapter_registry'
 
 module Faraday
@@ -27,7 +29,7 @@ class Handler
 
       attr_reader :name
 
-      def initialize(klass, *args, &block)
+      ruby2_keywords def initialize(klass, *args, &block)
         @name = klass.to_s
         REGISTRY.set(klass) if klass.respond_to?(:name)
         @args = args
@@ -89,7 +91,7 @@ def locked?
       @handlers.frozen?
     end
 
-    def use(klass, *args, &block)
+    ruby2_keywords def use(klass, *args, &block)
       if klass.is_a? Symbol
         use_symbol(Faraday::Middleware, klass, *args, &block)
       else
@@ -234,7 +236,7 @@ def is_adapter?(klass) # rubocop:disable Naming/PredicateName
       klass.ancestors.include?(Faraday::Adapter)
     end
 
-    def use_symbol(mod, key, *args, &block)
+    ruby2_keywords def use_symbol(mod, key, *args, &block)
       use(mod.lookup_middleware(key), *args, &block)
     end
 

spec/faraday/rack_builder_spec.rb

@@ -193,4 +193,29 @@ class Broken < Faraday::Middleware
       end
     end
   end
+
+  context 'when middleware is added with arguments' do
+    let(:conn) { Faraday::Connection.new {} }
+
+    class Dog < Faraday::Middleware
+      attr_accessor :name
+      def initialize(app, name)
+        super(app)
+        @name = name
+      end
+    end
+    class Cat < Faraday::Middleware
+      attr_accessor :name
+      def initialize(app, name:)
+        super(app)
+        @name = name
+      end
+    end
+    it 'adds a handler to construct middleware with options passed to use' do
+      subject.use Dog, 'Rex'
+      expect(subject.handlers.find { |handler| handler == Dog }.build.name).to eq('Rex')
+      subject.use Cat, name: 'Felix'
+      expect(subject.handlers.find { |handler| handler == Cat }.build.name).to eq('Felix')
+    end
+  end
 end