Support <scheme>.sub_plain for OpenFHE backend

[AlexanderViand-Intel]

Currently, we have bgv.sub_plain and ckks.sub_plain but no way to target them to OpenFHE

[asraa]

I think OpenFHE's EvalMul and EvalAdd are overloaded to support ciphertext-ciphertext and ciphertext-plaintext

[AlexanderViand-Intel]

I think OpenFHE's EvalMul and EvalAdd are overloaded to support ciphertext-ciphertext and ciphertext-plaintext

Yep, and those the pipeline already supports. But I'm not sure its supported for EvalSub in OpenFHE? If not, that just means we need to translate it to "negate plaintext, then add" (assuming that's the direction the op is defined in, which I don't recall right now 🙈 )

[asraa]

Oh! Gotcha - I think you're right about that and having some kind of rewrite/canonicalization pattern.