Implement MSC1708 (.well-known lookups for server routing) (#4489)

MANIFEST.in

changelog.d/4408.feature

changelog.d/4408.misc

changelog.d/4409.feature

changelog.d/4409.misc

changelog.d/4426.feature

changelog.d/4426.misc

changelog.d/4427.feature

changelog.d/4427.misc

changelog.d/4428.feature

changelog.d/4428.misc

changelog.d/4464.feature

changelog.d/4464.misc

changelog.d/4468.feature

changelog.d/4468.misc

changelog.d/4487.feature

changelog.d/4487.misc

changelog.d/4489.feature

synapse/http/federation/matrix_federation_agent.py

tests/http/__init__.py