Add support for controlling Matrix federation

roles/matrix-server/defaults/main.yml

@@ -86,6 +86,15 @@ matrix_synapse_event_cache_size: "100K"
 # - https://github.com/matrix-org/synapse/issues/3939
 matrix_synapse_cache_factor: 0.5
 
+# Controls whether Matrix Synapse will federate at all.
+# Disable this to completely isolate your server from the rest of the Matrix network.
+matrix_synapse_federation_enabled: true
+
+# A list of domain names that are allowed to federate with the given Matrix Synapse server.
+# An empty list value (`[]`) will also effectively stop federation, but if that's the desired
+# result, it's better to accomplish it by changing `matrix_synapse_federation_enabled`.
+matrix_synapse_federation_domain_whitelist: ~
+
 # A list of additional "volumes" to mount in the container.
 # This list gets populated dynamically based on Synapse extensions that have been enabled.
 # Contains definition objects like this: `{"src": "/outside", "dst": "/inside", "options": "rw|ro|slave|.."}