Pass module API to OIDC mapping provider (#16974)

As done for SAML mapping provider, let's pass the module API to the OIDC
one so the mapper can do more logic in its code.

docs/sso_mapping_providers.md

@@ -50,11 +50,13 @@ comment these options out and use those specified by the module instead.
 
 A custom mapping provider must specify the following methods:
 
-* `def __init__(self, parsed_config)`
+* `def __init__(self, parsed_config, module_api)`
    - Arguments:
      - `parsed_config` - A configuration object that is the return value of the
        `parse_config` method. You should set any configuration options needed by
        the module here.
+     - `module_api` - a `synapse.module_api.ModuleApi` object which provides the
+       stable API available for extension modules.
 * `def parse_config(config)`
     - This method should have the `@staticmethod` decoration.
     - Arguments: