Add admin API for logging in as a user (#8617)

changelog.d/8617.feature

docs/admin_api/user_admin_api.rst

synapse/api/auth_blocking.py

synapse/handlers/_base.py

synapse/handlers/auth.py

synapse/handlers/deactivate_account.py

synapse/handlers/message.py

synapse/handlers/profile.py

synapse/handlers/register.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/handlers/sync.py

synapse/module_api/__init__.py

synapse/rest/admin/__init__.py

synapse/rest/admin/rooms.py

synapse/rest/admin/users.py

synapse/rest/client/v2_alpha/sync.py

synapse/server_notices/server_notices_manager.py

synapse/storage/databases/main/registration.py

tests/api/test_auth.py