Add "supported_extensions" method to MechanismDriver

This method returns the supported plugin extensions, given a
set of them. Each mechanism driver will be able to implement each
own filtering method.

By default, the method will return the same set of extensions
without any filtering.

Related-Bug: #1888829

Change-Id: I6fd62f6f393b635e6710020adab736a09a142556

neutron_lib/plugins/ml2/api.py

@@ -455,6 +455,21 @@ class MechanismDriver(object, metaclass=abc.ABCMeta):
         """
         return []
 
+    def supported_extensions(self, extensions):
+        """Return the mechanism driver supported extensions
+
+        By default this method will return the same provided set, without any
+        filtering. In case any particular mechanism driver needs to filter out
+        any specific extension or supports only a reduced set of extensions,
+        this method should be override.
+
+        :param extensions: set of extensions supported by the instance that
+                           created this mechanism driver.
+        :returns: a set of the extensions currently supported by this
+                  mechanism driver
+        """
+        return extensions
+
 
 class _TypeDriverBase(object, metaclass=abc.ABCMeta):
 

releasenotes/notes/mechanism_driver_supported_extensions-67e1b0b763571ae9.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Add ``supported_extensions`` method to ``MechanismDriver``. This method
+    provides an standard API to implement, per mechanism driver, a way to
+    filter the supported extensions. By default, this method will return the
+    same set of extensions passed.