Support Coefficient Packing

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).