dilithium: Derive a public key from the private key

[aflox405]

I might be wrong here, but I believe it should be possible to derive a public key from an existing private key with dilithium.
That would be nice as it would improve key handling a lot. "Loosing" the public key would not require generating a completely new key pair. As such, it would be helpful if there was a method like

mode.DerivePublicKey(pk *PrivateKey) (PublicKey, err)

to facilitate this.