CR-02 Authorization Logic Missing in Critical Functions

[xoloki]

The coordinator and signer methods process_inbound_mesages() will receive messages
from signers or coordinator, and pass down the messages to other methods depending on
its current state and the message's type. However, this information does not suffice for
proper authorization.
For example, in frost.rs at the gather_public_shares() method the coordinator
inserts shares into self.dkg_public_shares for a signer_id without actually validating
these shares came (i.e., were signed) by a signer underlying this signer_id. Hence the
coordinator will add these shares for the signer_id and remove the signer_id from the
ids_to_wait vector even if this signer never sent his shares