Implement OpenID Connect-based login (#7256)

changelog.d/7256.feature

docs/dev/oidc.md

docs/sample_config.yaml

mypy.ini

synapse/app/homeserver.py

synapse/config/_base.pyi

synapse/config/homeserver.py

synapse/config/oidc_config.py

synapse/config/sso.py

synapse/handlers/auth.py

synapse/handlers/oidc_handler.py

synapse/http/client.py

synapse/python_dependencies.py

synapse/res/templates/sso_error.html

synapse/rest/client/v1/login.py

synapse/rest/oidc/__init__.py

synapse/rest/oidc/callback_resource.py

synapse/server.py

synapse/server.pyi

tests/handlers/test_oidc.py