[kerngen]: support ctxt-ptxt operations

[AlexanderViand-Intel]

The current set of kernels/supported operations does not include add_plain, sub_plain, mul_plain, negate, nor mul_const which arise when tracing an OpenFHE program. See also this comment here (internal link).

I noticed @christopherngutierrez already created bgv-add-plain-kernel, so hopefully we can get that merged into main + extend support to the other ops easily.

[faberga]

Hi @AlexanderViand-Intel, the basic Kergen operators do support cartesian operations over vectors of polynomials by providing the desired input type, without the need of specialized kernels as the legacy kernel generator did. Kergen is agnostic to the dialect that OpenFHE legacy traces use for labelling its operations. As a temporary remediation, while we complete the new trace infrastructure for OpenFHE, we can add the dialect conversion to the program mapper in a similar fashion as we did for SEAL.