feat: add Signature trait #2139

Job	Run time
docs	30s
features stable	2m 9s
clippy	46s
check no_std	51s
features stable --all-targets	3m 23s
fmt	12s
features nightly	1m 33s
features nightly --all-targets	2m 44s
miri --no-default-features	4m 44s
wasm	36s
miri	4m 48s
test ubuntu-latest stable --no-default-features	1m 18s
miri --all-features	7m 26s
test ubuntu-latest stable	1m 17s
test ubuntu-latest nightly --no-default-features	45s
test ubuntu-latest nightly	52s
test ubuntu-latest 1.79 --no-default-features	41s
test ubuntu-latest 1.79	39s
test windows-latest stable --no-default-features	4m 3s
test windows-latest stable	3m 30s
test windows-latest nightly --no-default-features	1m 48s
test windows-latest nightly	2m 1s
test windows-latest 1.79 --no-default-features	1m 59s
test windows-latest 1.79	1m 57s
test ubuntu-latest 1.79 --features json	43s
test windows-latest 1.79 --features json	1m 30s
test ubuntu-latest nightly --all-features	1m 12s
test windows-latest nightly --all-features	2m 25s
ci success	4s
	56m 26s

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