Add m.id_access_token to /versions unstable_features (MSC2264) (#5974)

Adds a flag to /versions' unstable_features section indicating that this Synapse understands what an id_access_token is, as per MSC2264.

Fixes #5927

Author: anoadragon453

changelog.d/5974.feature

@@ -0,0 +1 @@
+Add m.id_access_token to unstable_features in /versions as per MSC2264.

synapse/rest/client/versions.py

@@ -50,6 +50,9 @@ def on_GET(self, request):
                 # as per MSC1497:
                 "unstable_features": {
                     "m.lazy_load_members": True,
+                    # as per MSC2190, as amended by MSC2264
+                    # to be removed in r0.6.0
+                    "m.id_access_token": True,
                     # Advertise to clients that they need not include an `id_server`
                     # parameter during registration or password reset, as Synapse now decides
                     # itself which identity server to use (or none at all).