Add a module API to allow modules to edit push rule actions (#12406)

changelog.d/12406.feature

synapse/handlers/push_rules.py

synapse/module_api/__init__.py

synapse/module_api/errors.py

synapse/rest/client/push_rule.py

synapse/server.py

synapse/storage/databases/main/push_rule.py

tests/module_api/test_api.py