[FEA] large string support

[jlowe]

libcudf now supports string columns with more than 2GB of character data. There are many places in the C++ and Java code that assume offsets are 4 bytes, and large string support violates that assumption, so we disabled libcudf's large string support (see #2189). We need to find and update all of those places before enabling large-string support.

[jlowe]

rapidsai/cudf#16215 tracks the necessary changes in the cudf submodule.