Mirrors the C++ enum PlaintextEncodings in
pke/constants-defs.h.
Value
A named list of 5 integer scalars, each the value of the
OpenFHE C++ enumerator of the same name, naming how values are laid
out inside a plaintext polynomial: COEF_PACKED_ENCODING,
PACKED_ENCODING (integer SIMD slots), STRING_ENCODING and
CKKS_PACKED_ENCODING (approximate real or complex slots).
INVALID_ENCODING marks an unset value. This is the value reported
by get_encoding_type() on a Plaintext.