chore(consensus): blanket impl of `BlockHeader` for deref #4331

Job	Run time
doctest	58s
feature-checks	3m 22s
check no_std	17s
test ubuntu-latest stable --no-default-features	1m 30s
wasm-unknown	44s
test ubuntu-latest stable	1m 21s
wasm-wasi	1m 19s
cfg-check	25s
test ubuntu-latest stable --all-features	2m 20s
test ubuntu-latest nightly --no-default-features	1m 16s
clippy	37s
test ubuntu-latest nightly	1m 17s
docs	55s
test ubuntu-latest nightly --all-features	2m 2s
fmt	15s
test ubuntu-latest 1.79 --no-default-features	40s
test ubuntu-latest 1.79	31s
test windows-latest stable --no-default-features	4m 3s
test windows-latest stable	3m 55s
test windows-latest stable --all-features	5m 11s
test windows-latest nightly --no-default-features	3m 36s
test windows-latest nightly	3m 25s
test windows-latest nightly --all-features	4m 57s
test windows-latest 1.79 --no-default-features	2m 12s
test windows-latest 1.79	2m 39s
	49m 47s

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