Adds missing type hints to methods in the synapse.handlers
module and requires all methods to have type hints there.
This also removes the unused construct_auth_difference method
from the FederationHandler.
Applied a (slightly modified) patch from https://github.com/matrix-org/synapse/issues/9574.
As far as I understand this would allow the cookie set during the OIDC flow to work on deployments using public baseurls that do not sit at the URL path root.
