[FEA] Decompose null_count (and len) expressions as summing Literals on CPU in cudf_polars

From https://github.com/rapidsai/cudf/pull/22126#discussion_r3083332842, an alternative idea when decomposing a null count expression is to create `Literal` expressions and sum the `Literal.value`s on the CPU to avoid allocating GPU memory since the `Column`s already advertise their `.null_count` (i.e. a computation we don't necessarily need the GPU for?)

Possibly something can also apply for decomposing a `Len` expression since the `Column`s already advertise their `.size`

Not currently confident all the pieces are in place to do this today.