add(rpc): Adds `getblockheader` RPC method #2002

Job	Run time
Build CD Docker / Build images	1s
Get disk name	0s
Test CD default Docker config file / Test default-conf in Docker	1s
Test CD testnet Docker config file / Test default-conf in Docker	1s
Test CD custom Docker config file / Test custom-conf in Docker	1s
	4s

Provide feedback