Add --kty and --crv/--curve support to 'step ssh rekey'

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Issue details

Currently, step ssh rekey outputs an ECDSA key and there doesn't appear to be a way to specify --kty / --curve like with step ssh certificate or step ca rekey.

Why is this needed?

For consistency with the other commands that got updated as part of https://github.com/smallstep/cli/issues/477.