Allow SAML username provider plugins (#6411)

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+# SAML Mapping Providers
+
+A SAML mapping provider is a Python class (loaded via a Python module) that
+works out how to map attributes of a SAML response object to Matrix-specific
+user attributes. Details such as user ID localpart, displayname, and even avatar
+URLs are all things that can be mapped from talking to a SSO service.
+
+As an example, a SSO service may return the email address
+"john.smith@example.com" for a user, whereas Synapse will need to figure out how
+to turn that into a displayname when creating a Matrix user for this individual.
+It may choose `John Smith`, or `Smith, John [Example.com]` or any number of
+variations. As each Synapse configuration may want something different, this is
+where SAML mapping providers come into play.
+
+## Enabling Providers
+
+External mapping providers are provided to Synapse in the form of an external
+Python module. Retrieve this module from [PyPi](https://pypi.org) or elsewhere,
+then tell Synapse where to look for the handler class by editing the
+`saml2_config.user_mapping_provider.module` config option.
+
+`saml2_config.user_mapping_provider.config` allows you to provide custom
+configuration options to the module. Check with the module's documentation for
+what options it provides (if any). The options listed by default are for the
+user mapping provider built in to Synapse. If using a custom module, you should
+comment these options out and use those specified by the module instead.
+
+## Building a Custom Mapping Provider
+
+A custom mapping provider must specify the following methods:
+
+* `__init__(self, parsed_config)`
+   - 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.
+* `saml_response_to_user_attributes(self, saml_response, failures)`
+    - Arguments:
+      - `saml_response` - A `saml2.response.AuthnResponse` object to extract user
+                          information from.
+      - `failures` - An `int` that represents the amount of times the returned
+                     mxid localpart mapping has failed.  This should be used
+                     to create a deduplicated mxid localpart which should be
+                     returned instead. For example, if this method returns
+                     `john.doe` as the value of `mxid_localpart` in the returned
+                     dict, and that is already taken on the homeserver, this
+                     method will be called again with the same parameters but
+                     with failures=1. The method should then return a different
+                     `mxid_localpart` value, such as `john.doe1`.
+    - This method must return a dictionary, which will then be used by Synapse
+      to build a new user. The following keys are allowed:
+       * `mxid_localpart` - Required. The mxid localpart of the new user.
+       * `displayname` - The displayname of the new user. If not provided, will default to
+                         the value of `mxid_localpart`.
+* `parse_config(config)`
+    - This method should have the `@staticmethod` decoration.
+    - Arguments:
+        - `config` - A `dict` representing the parsed content of the
+          `saml2_config.user_mapping_provider.config` homeserver config option.
+           Runs on homeserver startup. Providers should extract any option values
+           they need here.
+    - Whatever is returned will be passed back to the user mapping provider module's
+      `__init__` method during construction.
+* `get_saml_attributes(config)`
+    - This method should have the `@staticmethod` decoration.
+    - Arguments:
+        - `config` - A object resulting from a call to `parse_config`.
+    - Returns a tuple of two sets. The first set equates to the saml auth
+      response attributes that are required for the module to function, whereas
+      the second set consists of those attributes which can be used if available,
+      but are not necessary.
+
+## Synapse's Default Provider
+
+Synapse has a built-in SAML mapping provider if a custom provider isn't
+specified in the config. It is located at
+[`synapse.handlers.saml_handler.DefaultSamlMappingProvider`](../synapse/handlers/saml_handler.py).