rpc: add `uncle_block_from_header` impl and test #4759

Job	Run time
check no_std	51s
doctest	2m 41s
feature-checks	2m 43s
clippy	1m 28s
wasm-unknown	2m 11s
test ubuntu-latest stable --no-default-features	2m 28s
wasm-wasi	2m 45s
cfg-check	37s
test ubuntu-latest stable	2m 29s
docs	1m 10s
test ubuntu-latest stable --all-features	2m 44s
fmt	15s
test ubuntu-latest nightly --no-default-features	1m 38s
test ubuntu-latest nightly	1m 41s
test ubuntu-latest nightly --all-features	2m 24s
test ubuntu-latest 1.79 --no-default-features	1m 23s
test ubuntu-latest 1.79	1m 38s
test windows-latest stable --no-default-features	2m 34s
test windows-latest stable	2m 36s
test windows-latest stable --all-features	2m 35s
test windows-latest nightly --no-default-features	2m 36s
test windows-latest nightly	2m 35s
test windows-latest nightly --all-features	2m 35s
test windows-latest 1.79 --no-default-features	2m 24s
test windows-latest 1.79	2m 25s
	51m 26s

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