Implement `Clone` for `SshSignature` #234

Job	Run time
macos-build-test-all	1m 27s
ubuntu-build-test-no-yubikey	48s
macos-build-test-experimental	1m 0s
macos-build-check-mozilla	1m 7s
macos-build-check-ctap2	49s
	5m 11s

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