cmd/geth: add 'dumpgenesis' command (#20191)

Adds the 'geth dumpgenesis' command, which writes the configured
genesis in JSON format to stdout. This provides a way to generate the
data (structure and content) that can then be used with the 'geth init'
command.

cmd/geth/main.go

@@ -205,6 +205,7 @@ func init() {
 		copydbCommand,
 		removedbCommand,
 		dumpCommand,
+		dumpGenesisCommand,
 		inspectCommand,
 		// See accountcmd.go:
 		accountCommand,