ssh-key: add `SshSig` serializing/deserializing using `serde` (#264) #633

Job	Run time
test (1.73.0)	2m 43s
no_std (1.73.0, thumbv7em-none-eabi)	57s
test (stable)	2m 29s
no_std (1.73.0, wasm32-unknown-unknown)	1m 3s
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	47s
no_std (stable, thumbv7em-none-eabi)	52s
build (x86_64-apple-darwin, macos-latest, stable)	31s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 25s
no_std (stable, wasm32-unknown-unknown)	50s
	11m 37s

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