ssh-key: add Algorithm::Other Signature support #418

Job	Run time
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	40s
build (x86_64-apple-darwin, macos-latest, stable)	52s
no_std (1.65.0, thumbv7em-none-eabi)	10m 13s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 13s
test (1.65.0)	26m 55s
no_std (1.65.0, wasm32-unknown-unknown)	9m 50s
test (stable)	20m 41s
no_std (stable, thumbv7em-none-eabi)	6m 58s
no_std (stable, wasm32-unknown-unknown)	7m 30s
minimal-versions / minimal-versions	48s
	1h 25m 40s

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