Add an unstable feature flag for separate add/bind 3pid APIs (#6044)

Add a m.separate_add_and_bind flag set to True. See MSC2290's Backward Compatibility section for details.

changelog.d/6044.feature

@@ -0,0 +1 @@
+Add an unstable feature flag for separate add/bind 3pid APIs.

synapse/rest/client/versions.py

@@ -63,6 +63,8 @@ class VersionsRestServlet(RestServlet):
                     # verification itself, there is no need to ask the user for `id_server` to
                     # be supplied.
                     "m.require_identity_server": False,
+                    # as per MSC2290
+                    "m.separate_add_and_bind": True,
                 },
             },
         )