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/modules.py

@@ -0,0 +1,49 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import Any, Dict, List, Tuple
+
+from synapse.config._base import Config, ConfigError
+from synapse.util.module_loader import load_module
+
+
+class ModulesConfig(Config):
+    section = "modules"
+
+    def read_config(self, config: dict, **kwargs):
+        self.loaded_modules: List[Tuple[Any, Dict]] = []
+
+        configured_modules = config.get("modules") or []
+        for i, module in enumerate(configured_modules):
+            config_path = ("modules", "<item %i>" % i)
+            if not isinstance(module, dict):
+                raise ConfigError("expected a mapping", config_path)
+
+            self.loaded_modules.append(load_module(module, config_path))
+
+    def generate_config_section(self, **kwargs):
+        return """
+            ## Modules ##
+
+            # Server admins can expand Synapse's functionality with external modules.
+            #
+            # See https://matrix-org.github.io/synapse/develop/modules.html for more
+            # documentation on how to configure or create custom modules for Synapse.
+            #
+            modules:
+                # - module: my_super_module.MySuperClass
+                #   config:
+                #       do_thing: true
+                # - module: my_other_super_module.SomeClass
+                #   config: {}
+            """