ssh-key: Implement SkEcdsaSha2NistP256 signature validation #387

Job	Run time
minimal-versions	4m 19s
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	49s
no_std (1.65.0, thumbv7em-none-eabi)	12m 22s
build (x86_64-apple-darwin, macos-latest, stable)	2m 59s
test (1.65.0)	19m 1s
no_std (1.65.0, wasm32-unknown-unknown)	12m 22s
test (stable)	12m 5s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 25s
no_std (stable, thumbv7em-none-eabi)	7m 0s
no_std (stable, wasm32-unknown-unknown)	7m 1s
	1h 19m 23s

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