Change network to interface in SimpleSigner

[nkryuchkov]

Changes:

• SimpleSigner expects an interface type for the network parameter instead of core.Network

ssvlabs/ssv#1308 introduces support of custom network configs where any value can be overridden. Currently, SimpleSigner expects the network parameter of the type core.Network which means that it supports only values stored in this repository. On any other values, it returns default zero-values or crashes.

Therefore, when the SSV node calls NewSimpleSigner on setting up a KeyManager, if the network name differs from supported, then SimpleSigner would not work as expected.

The SimpleSigner uses two core.Network's methods:

• EstimatedEpochAtSlot(slot phase0.Slot) phase0.Epoch
• EstimatedSlotAtTime(time int64) phase0.Slot

EstimatedEpochAtSlot calls only SlotsPerEpoch which returns 32 for any network. However, EstimatedSlotAtTime calls MinGenesisTime which returns 0 for unknown ones.

The PR changes the type of the network parameter to the interface below to allow the SSV node to just pass any network matching the interface.

type network interface {
	EstimatedEpochAtSlot(slot phase0.Slot) phase0.Epoch
	EstimatedSlotAtTime(time int64) phase0.Slot
}