Fix SSO on workers (#9271)

Fixes #8966.

* Factor out build_synapse_client_resource_tree

Start a function which will mount resources common to all workers.

* Move sso init into build_synapse_client_resource_tree

... so that we don't have to do it for each worker

* Fix SSO-login-via-a-worker

Expose the SSO login endpoints on workers, like the documentation says.

* Update workers config for new endpoints

Add documentation for endpoints recently added (#8942, #9017, #9262)

* remove submit_token from workers endpoints list

this *doesn't* work on workers (yet).

* changelog

* Add a comment about the odd path for SAML2Resource