rpc: ensure that (*Client).AddNetworkFee can properly calculate network fee for main Notary transaction

[AnnaShaleva]

Consider the following incomplete Notary main transaction signers:

1. Deployed contract with verify method
2. Multisig signer
3. Notary contract

We need to ensure that (*Client).AddNetworkFee is able to calculate the network fee for signers 1 and 2 properly.

The resulting transaction network fee (which is AddNetworkFee(...) + CalculateNotaryFee(...)) should be enough to complete the transaction and to send it to the chain.

[roman-khimov]

The problem is that deployed contract can demand any kind of invocation script. It can be empty script, it can have some signature inside, it can have multiple signatures inside, it can have hash/hashes, whatever. Invocation script obviously affects size/GAS spent on verification, so it's impossible to solve in generic way.

[AnnaShaleva]

@roman-khimov, I think that at least we can solve this problem for empty invocation scripts (which is what our AddNetworkFee does). And this solves the problem with extra GAS addition for NeoFS proxy-contract witness verification (see the comment in pkg/morph/client/notary.go, L21-23)

But with the changes from #1825 it's not hard to extend the functionality of the AddNeworkFee to work with non-empty invocation scripts.