Add strategy option for PKCE

v1.7.0 adds the option to specify that your strategy should utilize PKCE.

Simply add the pkce option to your strategy and set it to true:

option :pkce, true

By default, this uses the recommended code_challenge_method of "S256", but in the event that you want to use basic, or some other future code challenge method, you can specify them in your strategy as well:

option :pkce_options, {
  :code_challenge => proc { |verifier| verifier },
  :code_challenge_method => "basic",
}

Note that the code_challenge must be supplied in the form of something that responds to call.