signet: Add Signet::OAuth2::Client#initialize (#992)

Author: aruma256

gems/signet/0.19/_test/test.rb

@@ -8,3 +8,12 @@
 
 class MyClient < Signet::OAuth2::Client
 end
+
+client = Signet::OAuth2::Client.new({
+  :authorization_uri => 'https://example.com/authorization',
+  :token_credential_uri => 'https://example.com/token',
+  :client_id => 'my_client_id',
+  :client_secret => 'my_client_secret',
+  :scope => 'example',
+  :redirect_uri => 'https://example.com/oauth'
+})

gems/signet/0.19/signet.rbs

@@ -4,6 +4,7 @@ module Signet
 
   module OAuth2
     class Client
+      def initialize: (?Hash[Symbol, untyped] options) -> void
     end
   end
 end