[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.

Commencez par localiser les définitions des opérations prises en charge par kerngen et examiner la branche existante bgv-add-plain-kernel. Vérifiez comment add_plain, sub_plain, mul_plain, negate et mul_const apparaissent lors du tracing d’un programme OpenFHE, puis vérifiez que les kernels générés prennent en charge chacune de ces opérations.

Rédigé par le modèle d'indexation à partir du texte de l'issue.