Support Coefficient Packing

[AlexanderViand-Intel]

Probably requires a more explicit packing layout first (e.g., #913).

The OpenFHE backend offers MakeCoefPackedPlaintext and we've got lwe.polynomial_coefficient_encoding, but right now we have neither a openfhe.make_coef_packed_plaintext operation to codegen/emit, nor would the compiler know how to deal with such ciphertexts. For example, rotate takes on a very different meaning for such a ciphertext (multiply with $X$ as in CGGI, not the complex automorphism-based rotate used with most other packed ciphertexts).

[j2kun]

Is rotate even desirable for such encodings? While I'm happy to have support for coefficient packing, it may be safer to raise an error if someone tries to rotate a coefficient packed ciphertext.

[AlexanderViand-Intel]

Yeah, I think raising an error is a good starting point, though I assume we eventually want to support the "multiply with $X$" rotation on coefficient packed ciphertexts for CGGI-stuff/scheme switching anyway.