allow uploading keys for cross-signing

synapse/api/errors.py

synapse/handlers/device.py

synapse/handlers/e2e_keys.py

synapse/handlers/sync.py

synapse/rest/client/v2_alpha/keys.py

synapse/storage/__init__.py

synapse/storage/devices.py

synapse/storage/end_to_end_keys.py

synapse/storage/schema/delta/56/signing_keys.sql

synapse/types.py

tests/handlers/test_e2e_keys.py