Provide the support for specifying a signer in keytool -genkeypair

RFC 8410 section 10.2 provides an example of X25519 certificate using Ed25519 to sign an X25519 public key. The keytool -genkeypair command currently can not generate the key agreement certificate, such as X25519 certificate.
Add -signer option to keytool -genkeypair command so the signer’s private key can be used to sign and generate a key agreement certificate. Also, the keytool needs to be able to display the appropriate key size of an XDH key, instead of XDH key of unknown size.