Introduce account stub for actor-like packages of RPC client

[AnnaShaleva]

Is your feature request related to a problem? Please describe.

Sometimes users of rpcclient/actor package don't have the ability to sign the transaction (consider the case when dApp uses WaletConnect for transaction signing), but still have to construct it and relay. And thus, there's a need to split the transaction creation/signing processes. For such case we have account stubs provided by notary Actor, like that:

neo-go/pkg/rpcclient/notary/accounts.go

Lines 12 to 17 in 51ac153

	// FakeSimpleAccount creates a fake account belonging to the given public key.
	// It uses a simple signature contract and this account has SignTx that
	// returns no error, but at the same time adds no signature (it obviously can't
	// do that, so CanSign() returns false for it). Use this account for Actor when
	// simple signatures are needed to be collected.
	func FakeSimpleAccount(k *keys.PublicKey) *wallet.Account {

These stubs are useful not only in notary package, but also in other actors.

Describe the solution you'd like

We may think about moving notary accounts stub to a package of their own, so that they can be reused by all actors.

Describe alternatives you've considered

Create stubs for every actor.

Additional context

Inspired by config_wizard's question from Discord:

I realise that I am using wallet connect for signing payloads. As a result I think I need to use a signer or an actor and an unsigned transaction to then get signed with wallet connect? I think this way using an account type, requires me to have the private key? I also guess it then becomes a two step operation (signing, receving signed)?

I have wallet connect all working and set up using NeoFS payloads (signing bearer tokens etc) so I reckon i know how to do that part, just wondering what the easiest way to create the transaction but export its data to sign is?
i guess i might have to manually create a transaction, but is there a way to get the transaction from above (using the helpers etc) before it gets signed/transferred and then call the sender manually once i have a signed payload?

[roman-khimov]

Makes sense and we have almost everything needed already.