core, eth: split eth package, implement snap protocol (#21482)

This commit splits the eth package, separating the handling of eth and snap protocols. It also includes the capability to run snap sync (https://github.com/ethereum/devp2p/blob/master/caps/snap.md) , but does not enable it by default. 

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Martin Holst Swende <martin@swende.se>

internal/ethapi/backend.go

@@ -41,7 +41,6 @@ import (
 type Backend interface {
 	// General Ethereum API
 	Downloader() *downloader.Downloader
-	ProtocolVersion() int
 	SuggestPrice(ctx context.Context) (*big.Int, error)
 	ChainDb() ethdb.Database
 	AccountManager() *accounts.Manager