Speed up cache size command (#17015)
## Summary
`uv cache size` can be quite slow. Here i use
https://github.com/sharkdp/diskus to walk the cache directory with in
multiple threads.
Add cli option to set the number of threads and default to `
std::thread::available_parallelism()` or 1.
## Test Plan
Added cli statement with info log test.
I believe this is a fair test, where i set cache dir to a large
directory.
```bash
matthew@matthew-main ~/develop/personal/uv [14:17:50] [±cache-size-speed-up ✓▴]
> $ uv cache size --preview-features cache-size -H --cache-dir ~/develop/ [±cache-size-speed-up ✓▴]
75.7GiB
matthew@matthew-main ~/develop/personal/uv [14:18:24]
> $ hyperfine 'uv cache size --preview-features cache-size -H --cache-dir ~/develop/' 'target/debug/uv cache size --preview-features cache-size -H --cache-dir ~/develop/' [±cache-size-speed-up ✓▴]
Benchmark 1: uv cache size --preview-features cache-size -H --cache-dir ~/develop/
Time (mean ± σ): 1.059 s ± 0.014 s [User: 0.171 s, System: 0.884 s]
Range (min … max): 1.048 s … 1.097 s 10 runs
Benchmark 2: target/debug/uv cache size --preview-features cache-size -H --cache-dir ~/develop/
Time (mean ± σ): 413.8 ms ± 17.1 ms [User: 5789.2 ms, System: 1682.0 ms]
Range (min … max): 386.3 ms … 441.6 ms 10 runs
Summary
target/debug/uv cache size --preview-features cache-size -H --cache-dir ~/develop/ ran
2.56 ± 0.11 times faster than uv cache size --preview-features cache-size -H --cache-dir ~/develop/
```
