consensus: implement `Arbitrary` for `Header` #3736

Job	Run time
test ubuntu-latest stable --no-default-features	2m 39s
test ubuntu-latest stable	2m 27s
test ubuntu-latest stable --all-features	2m 46s
doctest	2m 34s
wasm-unknown	2m 56s
test ubuntu-latest nightly --no-default-features	1m 55s
test ubuntu-latest nightly	2m 8s
wasm-wasi	3m 47s
test ubuntu-latest nightly --all-features	2m 19s
test ubuntu-latest 1.78 --no-default-features	1m 26s
test ubuntu-latest 1.78	1m 40s
test windows-latest stable --no-default-features	5m 43s
cfg-check	23s
test windows-latest stable	5m 59s
test windows-latest stable --all-features	7m 8s
clippy	1m 17s
test windows-latest nightly --no-default-features	5m 0s
test windows-latest nightly	4m 30s
test windows-latest nightly --all-features	5m 27s
docs	1m 21s
test windows-latest 1.78 --no-default-features	3m 58s
test windows-latest 1.78	3m 30s
fmt	10s
feature-checks	7m 45s
no-std	18s
	1h 19m 6s

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