Add disk cache to prefetch benchmark

[dannycjones]

This change adds the ability to configure the prefetch benchmark with a disk cache. The configuration for the benchmark is slightly different from the main mount-s3 CLI - we allow the cache cleanup to be turned off to test warm caches. Additionally, the eviction mechanism is turned off and the cache is unbounded.

This patch includes some small refactors, to allow us to share defaults across benchmarks and the main CLI such that they are kept in sync.

It also introduces CLI headings for the help, to make it clearer to users (us) which options relate to which components: S3 client, prefetcher, etc..

Does this change impact existing behavior?

Benchmark change only.

Does this change need a changelog entry? Does it require a version change?

No, benchmark change only.

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[dannycjones]

New help output for prefetch_benchmark:

Run workloads against the prefetcher component of Mountpoint. Fetched data is discarded.

Usage: prefetch_benchmark [OPTIONS] <BUCKET> <S3_KEY>

Arguments:
  <BUCKET>  S3 bucket name containing the S3 object to fetch
  <S3_KEY>  S3 object key to fetch

Options:
      --region <AWS_REGION>  AWS region of the bucket [default: us-east-1]
  -h, --help                 Print help

S3 client options:
      --maximum-throughput-gbps <Gib/s>
          Target throughput in gibibits per second
      --crt-memory-limit-gib <GiB>
          Override value for CRT memory limit in gibibytes
      --part-size <BYTES>
          Part size for multi-part GET in bytes
      --bind <NETWORK_INTERFACE>
          One or more network interfaces to use when accessing S3. Requires Linux 5.7+ or running as root.

Prefetcher options:
      --max-memory-target <MiB>    Maximum memory usage target for Mountpoint's memory limiter [default: 95% of total system memory]
      --read-size <BYTES>          Size of read requests requests to the prefetcher [default: 131072]
      --disk-cache <DIRECTORY>     Enable caching of object content to the given directory. The disk cache has no max size for this benchmark, so you must ensure there is enough available space. [aliases: --cache]
      --disk-cache-cleanup <MODE>  Configure how the benchmark cleans the data cache directory. [default: every-iteration] [possible values: never, every-iteration]

Benchmark options:
      --iterations <N>  Number of times to download the S3 object [default: 1]
      --downloads <N>   Number of concurrent downloads [default: 1]