consensus: implement `seal` helper for `Header` #3864

Job	Run time
doctest	1m 55s
no-std	20s
wasm-unknown	1m 40s
wasm-wasi	2m 31s
test ubuntu-latest stable --no-default-features	1m 53s
cfg-check	32s
test ubuntu-latest stable	1m 47s
clippy	1m 1s
test ubuntu-latest stable --all-features	2m 22s
docs	57s
test ubuntu-latest nightly --no-default-features	1m 6s
feature-checks	3m 5s
test ubuntu-latest nightly	1m 15s
fmt	15s
test ubuntu-latest nightly --all-features	1m 58s
test ubuntu-latest 1.79 --no-default-features	32s
test ubuntu-latest 1.79	38s
test windows-latest stable --no-default-features	4m 16s
test windows-latest stable	4m 16s
test windows-latest stable --all-features	4m 59s
test windows-latest nightly --no-default-features	3m 0s
test windows-latest nightly	3m 15s
test windows-latest nightly --all-features	4m 35s
test windows-latest 1.79 --no-default-features	2m 13s
test windows-latest 1.79	3m 16s
	53m 37s

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