add(rpc): Adds `getblockheader` RPC method #2037

Job	Run time
Generate crates matrix	18s
Check crates matrix	3s
Build zebrad crate	2m 27s
Build zebra-utils crate	1m 8s
Build zebra-test crate	1m 31s
Build zebra-state crate	2m 28s
Build zebra-script crate	1s
Build zebra-scan crate	1s
Build zebra-rpc crate	1s
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
	8m 6s

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