add(rpc): Adds `getblockheader` RPC method #2038

Job	Run time
Generate crates matrix	21s
Check crates matrix	3s
Build zebrad crate	38m 32s
Build zebra-utils crate	16m 47s
Build zebra-test crate	1m 33s
Build zebra-state crate	22m 57s
Build zebra-script crate	3m 37s
Build zebra-scan crate	26m 40s
Build zebra-rpc crate	32m 55s
Build zebra-node-services crate	3m 15s
Build zebra-network crate	6m 49s
Build zebra-grpc crate	14m 59s
Build zebra-consensus crate	16m 32s
Build zebra-chain crate	4m 53s
Build tower-fallback crate	1m 9s
Build tower-batch-control crate	1m 32s
Open or update issues for building crates individually failures	1s
	3h 12m 35s

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