Mirrors the C++ enum SCHEME in pke/scheme/scheme-id.h. The R-side
name SchemeId matches the upstream header filename and avoids
colliding with a potential future Scheme S7 class.
Value
A named list of 4 integer scalars, each the value of the
OpenFHE C++ enumerator of the same name, identifying the encryption
scheme a context implements: CKKSRNS_SCHEME, BFVRNS_SCHEME or
BGVRNS_SCHEME, with INVALID_SCHEME for an unset value. This is
the value reported by get_scheme() on any CCParams object.