A node should be able to participate in BLS multi-signature only if it has a valid proof of possession

Once all Stewards send a NODE txn with proof of possession, we should allow a Node to participate in BLS multi-signature only if it has a BLS key with a proof of possession.

Acceptance criteria

• Check that there is a valid proof of possession when creating a registry of BLS keys (see `BlsKeyRegister`)
• Add tests that Nodes without PoP can not participate in BLS multi-signature.
• The code may be disabled for a time being (until all Stewards provide a PoP).