Add SignedData func

[smallhive]

closes #510

Added func to known places with Sign method, plus for some places when I think it would be useful. Of course, in some way it is almost similar to Marshal func, but for consistency called it SignedData.

If I missed any place, just mention it

[smallhive]

what about object.Object?

Nothing, it has a different API for signing with different names. I tend to think we may rewrite it to be compatible with SignedComponent interface. I did these changes in the last commit

type SignedComponent interface { SignedData() []byte Sign(neofscrypto.Signer) error VerifySignature() bool }

I've added the suggested test, but we had some changes in code, according to this

[cthulhu-rider]

SignedData returns data that _would be signed_ breaks me a little. Mabe SignedData returns the data that is signed after a [Token.Sign] call?

// SignedData returns signed data of the T.
//
// See also [T.Sign].

?

[smallhive]

Updated. Make two test interfaces/functions for each user.Signer and neofscrypto.Signer. But Object functions I renamed to be compatible with new test interface and more important with other signable components

[smallhive]

No problem with the signer it is OK. Problem with checking functions

func SignedDataComponent(tb testing.TB, signer neofscrypto.Signer, cmp SignedComponent)

func SignedDataComponentUser(tb testing.TB, signer user.Signer, cmp SignedComponentUserSigner)

They don't want to accept the wrong types for cmp parameter.

Cannot use 'val' (type *session.Container) as the type SignedComponent Type does not implement 'SignedComponent'
need the method: Sign(neofscrypto.Signer) error 
have the method: Sign(signer user.Signer) error

[smallhive]

That is why we have two interfaces and two functions to check them

[cthulhu-rider]

need the method: Sign(neofscrypto.Signer) error 
have the method: Sign(signer user.Signer) error

needed method is more generic and may be implemented in-place