Extract signing root function for all eth2 types

[corverroos]

Problem to be solved

We sign eth2 structs in validatormock and in many tests. Signing requires calculating the domain wrapped signature root of the struct. We currently do this manually in many places. It is time to DRY.

Proposed solution

Create a function in eth2util/signing that calculates signing roots for all unsigned eth2 types, following the example of VerifySignedData.

func UnsignedRoot(ctx context.Context, eth2Cl Eth2Provider, unsigned interface{}) ([32]byte, error) {

Refactor all manual instances to use this code.