auth+proxy: extend Authenticator methods with target service name

The target service name remains unused in its current form, but will be
required in order to verify that an incoming request with an LSAT
attached is authorized to access the service being attempted. We can
derive this from the request's host field, but we choose to extend the
methods with the additional parameter in order to prevent parsing the
host field again to determine which service is being accessed.

auth/authenticator_test.go

@@ -141,7 +141,7 @@ func TestLsatAuthenticator(t *testing.T) {
 	}
 
 	for _, testCase := range headerTests {
-		result := a.Accept(testCase.header)
+		result := a.Accept(testCase.header, "test")
 		if result != testCase.result {
 			t.Fatalf("test case %s failed. got %v expected %v",
 				testCase.id, result, testCase.result)