cmd/geth: enable DNS discovery by default (#20660)

* node: expose config in service context

* eth: integrate p2p/dnsdisc

* cmd/geth: add some DNS flags

* eth: remove DNS URLs

* cmd/utils: configure DNS names for testnets

* params: update DNS URLs

* cmd/geth: configure mainnet DNS

* cmd/utils: rename DNS flag and fix flag processing

* cmd/utils: remove debug print

* node: fix test

eth/config.go

@@ -95,6 +95,10 @@ type Config struct {
 	NetworkId uint64 // Network ID to use for selecting peers to connect to
 	SyncMode  downloader.SyncMode
 
+	// This can be set to list of enrtree:// URLs which will be queried for
+	// for nodes to connect to.
+	DiscoveryURLs []string
+
 	NoPruning  bool // Whether to disable pruning and flush everything to disk
 	NoPrefetch bool // Whether to disable prefetching and only load state on demand
 
@@ -156,8 +160,8 @@ type Config struct {
 	CheckpointOracle *params.CheckpointOracleConfig `toml:",omitempty"`
 
 	// Istanbul block override (TODO: remove after the fork)
-	OverrideIstanbul *big.Int
+	OverrideIstanbul *big.Int `toml:",omitempty"`
 
 	// MuirGlacier block override (TODO: remove after the fork)
-	OverrideMuirGlacier *big.Int
+	OverrideMuirGlacier *big.Int `toml:",omitempty"`
 }