Use `SignKey` instead of `KeyPair` in Haskell signing

[annenkov]

Task description

The function signatures BlockSignature.sign and SignatureScheme.sign have KeyPair as a parameter. The interface of the ed25519-dalek library is different in v2: the verification key is not part of the API. For that reason, it doesn't make sense to pass KeyPair, since it's sufficient to pass SignKey .

These changes need to be propagated to concordium-node and concordium-client