OpenSSH supports a new SSHSIG format, which allows for the signing of any data using an OpenSSH key.
This is supported on the CLI via ssh-keygen -Y sign (and associated -Y friends)
but, support for SSHSIG is not implemented in
x/crypto.
I don't have the time to handle upstreaming this to x/crypto, but I'd very
much welcome someone upstreaming this (and deprecating this module), I'm happy
to sign whatever is needed to help with that, including relicensing for inclusion
into Go's x/crypto. Until then, this can do the work of signing and verification.