add(rpc): Adds `getblockheader` RPC method #2041

Job	Run time
Generate crates matrix	22s
Check crates matrix	4s
Build zebrad crate	18m 23s
Build zebra-utils crate	16m 26s
Build zebra-test crate	1m 35s
Build zebra-state crate	18m 25s
Build zebra-script crate	3m 21s
Build zebra-scan crate	13m 8s
Build zebra-rpc crate	1m 35s
Build zebra-node-services crate	1s
Build zebra-network crate	1s
Build zebra-grpc crate	1s
Build zebra-consensus crate	1s
Build zebra-chain crate	1s
Build tower-fallback crate	1s
Build tower-batch-control crate	1s
Open or update issues for building crates individually failures	1s
	1h 13m 27s

Provide feedback