add(rpc): Adds `getblockheader` RPC method #2042

Job	Run time
Generate crates matrix	12s
Check crates matrix	2s
Build zebrad crate	37m 43s
Build zebra-utils crate	17m 2s
Build zebra-test crate	1m 34s
Build zebra-state crate	22m 12s
Build zebra-script crate	3m 26s
Build zebra-scan crate	26m 49s
Build zebra-rpc crate	34m 30s
Build zebra-node-services crate	3m 18s
Build zebra-network crate	7m 14s
Build zebra-grpc crate	14m 46s
Build zebra-consensus crate	17m 19s
Build zebra-chain crate	4m 53s
Build tower-fallback crate	1m 8s
Build tower-batch-control crate	1m 33s
Open or update issues for building crates individually failures	0s
	3h 13m 41s

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