eth/catalyst: add catalyst API prototype (#22641)

This change adds the --catalyst flag, enabling an RPC API for eth2 integration.
In this initial version, catalyst mode also disables all peer-to-peer networking.

Co-authored-by: Mikhail Kalinin <noblesse.knight@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>

consensus/ethash/consensus.go

@@ -315,6 +315,8 @@ func (ethash *Ethash) CalcDifficulty(chain consensus.ChainHeaderReader, time uin
 func CalcDifficulty(config *params.ChainConfig, time uint64, parent *types.Header) *big.Int {
 	next := new(big.Int).Add(parent.Number, big1)
 	switch {
+	case config.IsCatalyst(next):
+		return big.NewInt(1)
 	case config.IsMuirGlacier(next):
 		return calcDifficultyEip2384(time, parent)
 	case config.IsConstantinople(next):