Expose `sigHash` for `SetCodeAuthorization`

[dkatzan]

Rationale

This package can be used for transaction handling, without using it for it's signing capabilities
currently it's easy to use this package for serializing transaction, but using a different signing method\package etc
every transaction type exposes a Hash function which calculates and returns the hash to be signed by the sender

for new set_code transaction, such a Hash function also exists, but that assumes the Authorization list were already signed and included in the transaction, but there is no way currently to get the Hash that should be signed for the authorization

Implementation

I propose renaming:
func (a *SetCodeAuthorization) sigHash() common.Hash
into
func (a *SetCodeAuthorization) Hash() common.Hash