[kerngen]: support ctxt-ptxt operations

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)](https://github.com/IntelLabs/HERACLES-openfhe-development/pull/85).

I noticed @christopherngutierrez already created [bgv-add-plain-kernel](https://github.com/IntelLabs/hec-p-isa-tools/tree/christopherngutierrez/bgv-add-plain-kernel), so hopefully we can get that merged into main + extend support to the other ops easily.

Start by locating kerngen's supported-operation definitions and reviewing the existing bgv-add-plain-kernel branch. Check how add_plain, sub_plain, mul_plain, negate, and mul_const arise when tracing an OpenFHE program, then verify that generated kernels support each operation.

Escrito por el modelo de indexación a partir del texto del issue.