Support PHP generic services (#3269)

* Add php_generic_services option

* Generate PHP generic services

* Respect namespaces for generated PHP services

* Test PHP generated services

* Rename PHP generator service method doc comment function

* Correct phpdoc service method case

* Test namespaced PHP generic services

* Always use the FQCN for PHP generic service input/output

* Add generated_service_test to php test.sh

* Add php service test protos to CI

* Add php service files to php_EXTRA_DIST

* Use Interface suffix for php generic services
diff --git a/php/phpunit.xml b/php/phpunit.xml
index 7cb1e2a..637467b 100644
--- a/php/phpunit.xml
+++ b/php/phpunit.xml
@@ -10,6 +10,7 @@
       <file>tests/generated_phpdoc_test.php</file>
       <file>tests/map_field_test.php</file>
       <file>tests/well_known_test.php</file>
+      <file>tests/generated_service_test.php</file>
     </testsuite>
   </testsuites>
 </phpunit>