[FEA] Provide a unified config API for all I/O controls currently managed with environment variables

**Is your feature request related to a problem? Please describe.**
A large number of cuIO's tuning knobs are currently only exposed as environment variables. That makes them difficult to tweak programmatically, Moreover, not all of them have historically been reread, so only their initial value controls behavior, meaning that iterative runs cannot be modified live.

**Describe the solution you'd like**
libcudf should expose a config API for all of these settings. That will allow engines built on top of libcudf to benchmark different options and determine optimal configurations.

Start by inventorying cuIO tuning knobs currently exposed as environment variables and reviewing how libcudf reads them. Define the unified configuration surface and live-update behavior; done means engines can programmatically set and benchmark all of these I/O options, including settings that previously only took effect at startup.

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