chore(consensus): move `Header::size` into trait `BlockHeader` #4303

Job	Run time
check no_std	30s
clippy	50s
docs	1m 56s
cfg-check	1m 14s
doctest	1m 21s
feature-checks	5m 49s
fmt	11s
test ubuntu-latest stable --no-default-features	1m 25s
wasm-wasi	2m 4s
wasm-unknown	1m 6s
test ubuntu-latest stable	1m 23s
test ubuntu-latest stable --all-features	2m 17s
test ubuntu-latest nightly --no-default-features	2m 29s
test ubuntu-latest nightly	2m 32s
test ubuntu-latest nightly --all-features	2m 52s
test ubuntu-latest 1.79 --no-default-features	52s
test ubuntu-latest 1.79	56s
test windows-latest stable --no-default-features	5m 25s
test windows-latest stable	4m 26s
test windows-latest stable --all-features	4m 58s
test windows-latest nightly --no-default-features	5m 48s
test windows-latest nightly	6m 0s
test windows-latest nightly --all-features	6m 6s
test windows-latest 1.79 --no-default-features	2m 37s
test windows-latest 1.79	2m 31s
	1h 7m 38s

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