feat: add MaybeCancunPayloadFields::as_ref #2204

Job	Run time
cfg-check	1m 15s
docs	2m 5s
doctest	3m 22s
clippy	1m 42s
feature-checks	6m 42s
fmt	19s
test ubuntu-latest stable --no-default-features	2m 51s
test ubuntu-latest stable	2m 41s
wasm-unknown	2m 13s
wasm-wasi	3m 26s
test ubuntu-latest nightly --no-default-features	2m 51s
no-std	41s
test ubuntu-latest nightly	2m 42s
test ubuntu-latest 1.76 --no-default-features	1m 51s
test ubuntu-latest 1.76	1m 49s
test windows-latest stable --no-default-features	3m 14s
test windows-latest stable	4m 36s
test windows-latest nightly --no-default-features	6m 18s
test windows-latest nightly	6m 20s
test windows-latest 1.76 --no-default-features	3m 51s
test windows-latest 1.76	1m 53s
test ubuntu-latest stable --all-features	3m 9s
test ubuntu-latest nightly --all-features	3m 7s
	1h 8m 58s

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