Add annotation to define the type of key

[maraino]

What would you like to be added

Add a new annotation that allows the user to get an RSA/EdDSA instead of an ECDSA certificate.

Why this is needed

See https://github.com/smallstep/autocert/issues/4#issuecomment-537698075

[JackSinclairT]

we could really use this annotation as we rely on PKCS12 certificates in our scenario. If we could specify that in an annotation, that would make things ALOT easier!

[maraino]

@JackSinclairT With the current version, using custom images for the bootstrapper and renewer you can create certificates with different algorithms, and convert them to PKCS12.

For example, a new image with a bootstrapper.sh running step ca certificate with the flag --kty RSA will generate RSA keys (2048bits by default). And step certificate p12 site.p12 site.crt site.key can be added to convert the cert to PKCS12.

And a custom image of the renewer can be modified to convert the certificate to PKCS12 on each renewal:

step ca renew --daemon --exec "step certificate p12 site.p12 $CRT $KEY" $CRT $KEY