add(rpc): Adds `getblockheader` RPC method #2100

Job	Run time
Generate crates matrix	8s
Check crates matrix	2s
Build zebrad crate	37m 19s
Build zebra-utils crate	16m 43s
Build zebra-test crate	1m 26s
Build zebra-state crate	23m 4s
Build zebra-script crate	4m 1s
Build zebra-scan crate	27m 9s
Build zebra-rpc crate	34m 6s
Build zebra-node-services crate	3m 6s
Build zebra-network crate	6m 54s
Build zebra-grpc crate	15m 6s
Build zebra-consensus crate	17m 43s
Build zebra-chain crate	4m 51s
Build tower-fallback crate	58s
Build tower-batch-control crate	1m 26s
Open or update issues for building crates individually failures	0s
	3h 14m 2s

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