Introduce new `TransactionWithSignature` interface

Right now all transactions implement Signature() method, but we don't need it for L1Handler and Deploy transactions (right now they return empty slice).
We already faced some issues with it, here is an example:

if txSignature := transaction.Signature(); len(txSignature) > 0 {
   digest.Update(txSignature...)
} else {
   digest.Update(&felt.Zero)
}

it was rewritten as:

switch transaction.(type) {
  case *DeployTransaction, *L1HandlerTransaction:
     digest.Update(&felt.Zero)
  default:
    digest.Update(transaction.Signature()...)
}

but code above may lead to more questions: why we're specifying exactly these transactions? is it specific to this function only or applicable to entire codebase?

It can be solved by introducing new interface type TransactionWithSignature (name only for example, I'm pretty sure there is a better naming):

type TransactionWithSignature interface {
   Signature() []*felt.Felt
}

which can be used in example above as:

switch transaction.(type) {
case TransactionWithSignature:
     digest.Update(transaction.Signature()...)
default:
     digest.Update(&felt.Zero)
}

Start by tracing the transaction implementations and the call sites that invoke Signature(), especially the digest-update example in the issue. Confirm which transaction types provide signatures and define the interface boundary; done means callers no longer need to special-case concrete transaction types.