Add forward-compatible #auth for SASL mechanisms

A fully SASL-compatible API must first find the authenticator for the
mechanism and then delegate any and all arbitrary parameters to that
authenticator.  Practically, that means that the mechanism name must
either be the first positional parameter or a keyword parameter, and
then every other parameter can be forwarded.  `Net::SMTP#auth` does this.

For backward compatibility, `Net::SMTP#authenticate` is unchanged.

lib/net/smtp.rb

@@ -856,6 +856,29 @@ def authenticate(user, secret, authtype = DEFAULT_AUTH_TYPE)
       critical { authenticator.auth(user, secret) }
     end
 
+    # call-seq:
+    #   auth(type = DEFAULT_AUTH_TYPE, ...)
+    #   auth(type: DEFAULT_AUTH_TYPE, **kwargs, &block)
+    #
+    # All arguments besides +mechanism+ are forwarded directly to the
+    # authenticator.  Alternatively, +mechanism+ can be provided by the +type+
+    # keyword parameter.  Positional parameters cannot be used with +type+.
+    #
+    # Different authenticators take different options, but common options
+    # include +authcid+ for authentication identity, +authzid+ for authorization
+    # identity, +username+ for either "authentication identity" or
+    # "authorization identity" depending on the +mechanism+, and +password+.
+    def auth(authtype = nil, *args, type: nil, **kwargs, &blk)
+      type ||= authtype || DEFAULT_AUTH_TYPE
+      authenticator = Authenticator.auth_class(type).new(self)
+      if kwargs.empty?
+        # TODO: remove this, unless it is needed for 2.6/2.7/3.0 compatibility
+        critical { authenticator.auth(*args, &blk) }
+      else
+        critical { authenticator.auth(*args, **kwargs, &blk) }
+      end
+    end
+
     private
 
     def check_auth_method(type)

test/net/smtp/test_smtp.rb

@@ -110,6 +110,21 @@ def test_auth_plain
       smtp = Net::SMTP.start 'localhost', server.port
       assert smtp.authenticate("account", "password", :plain).success?
       assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("PLAIN", "account", "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth(type: "PLAIN", username: "account", secret: "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("PLAIN", username: "account", password: "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
     end
 
     def test_unsucessful_auth_plain
@@ -120,10 +135,20 @@ def test_unsucessful_auth_plain
       assert_equal "535", err.response.status
     end
 
+    def test_auth_cram_md5
+      server = FakeServer.start(auth: 'CRAM-MD5')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth(:cram_md5, "account", password: "password").success?
+    end
+
     def test_auth_login
       server = FakeServer.start(auth: 'login')
       smtp = Net::SMTP.start 'localhost', server.port
       assert smtp.authenticate("account", "password", :login).success?
+
+      server = FakeServer.start(auth: 'login')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("LOGIN", username: "account", secret: "password").success?
     end
 
     def test_unsucessful_auth_login