cmd, eth, internal, les: add txfee cap (#21212)

* cmd, eth, internal, les: add gasprice cap

* cmd/utils, eth: add default value for gasprice cap

* all: use txfee cap

* cmd, eth: add fix

* cmd, internal: address comments

internal/ethapi/backend.go

@@ -45,7 +45,8 @@ type Backend interface {
 	ChainDb() ethdb.Database
 	AccountManager() *accounts.Manager
 	ExtRPCEnabled() bool
-	RPCGasCap() *big.Int // global gas cap for eth_call over rpc: DoS protection
+	RPCGasCap() *big.Int  // global gas cap for eth_call over rpc: DoS protection
+	RPCTxFeeCap() float64 // global tx fee cap for all transaction related APIs
 
 	// Blockchain API
 	SetHead(number uint64)