agent:ssh: Fix RSA signature handling for newer spec.
* agent/command-ssh.c (SPEC_FLAG_WITH_FIXEDLENGTH): New. (struct ssh_key_type_spec): Add keysize field. (ssh_signature_encoder_rsa): Support the fixed length signature for RSA in RFC-8332. (ssh_handler_sign_request): Enable SPEC_FLAG_WITH_FIXEDLENGTH for rsa-sha2-256 and rsa-sha2-512. Set up keysize field for those.
- GnuPG-bug-id: T7882
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>