Add environment variables for I/O thread pool and slice sizes

[vuule]

Modified getenv_or to allow the default value of any type.
Adds environment variables:

• LIBCUDF_CUFILE_THREAD_COUNT: integer, number of threads in the pool used for I/O (default 16);
• LIBCUDF_CUFILE_SLICE_SIZE: integer, maximum size of cuFile reads/writes in bytes (default 4MB).

[codecov]

Codecov Report

Merging #10218 (15f1daf) into branch-22.04 (a7d88cd) will increase coverage by 0.05%.
The diff coverage is 0.00%.

❗ Current head 15f1daf differs from pull request most recent head 71566b7. Consider uploading reports for the commit 71566b7 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.04   #10218      +/-   ##
================================================
+ Coverage         10.42%   10.47%   +0.05%     
================================================
  Files               119      122       +3     
  Lines             20603    20487     -116     
================================================
- Hits               2148     2147       -1     
+ Misses            18455    18340     -115     

Impacted Files	Coverage Δ
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/main.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/comm/gpuarrow.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/_base_index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/datetime.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <0.00%> (ø)
... and 42 more

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[vuule]

@gpucibot merge