Standardise the module interface (#10062)

This PR adds a common configuration section for all modules (see docs). These modules are then loaded at startup by the homeserver. Modules register their hooks and web resources using the new `register_[...]_callbacks` and `register_web_resource` methods of the module API.

synapse/config/spam_checker.py

@@ -42,18 +42,3 @@ class SpamCheckerConfig(Config):
                 self.spam_checkers.append(load_module(spam_checker, config_path))
         else:
             raise ConfigError("spam_checker syntax is incorrect")
-
-    def generate_config_section(self, **kwargs):
-        return """\
-        # Spam checkers are third-party modules that can block specific actions
-        # of local users, such as creating rooms and registering undesirable
-        # usernames, as well as remote users by redacting incoming events.
-        #
-        spam_checker:
-           #- module: "my_custom_project.SuperSpamChecker"
-           #  config:
-           #    example_option: 'things'
-           #- module: "some_other_project.BadEventStopper"
-           #  config:
-           #    example_stop_events_from: ['@bad:example.com']
-        """