feat: expose backpressure and batching options in the perf client

[merlimat]

Motivation

The client builder has maxPendingBytes, maxWriteBatchesInFlight, maxReadBatchesInFlight and batchingThreads to control backpressure and batching, but the perf client offers no way to set them, so they cannot be tuned or compared when benchmarking.

Modifications

• Add four flags to PerfArguments, passed through to the builder in PerfClient. Their defaults come from the OxiaClientBuilderImpl constants:

  Flag	Builder option	Default
  --max-pending-bytes	maxPendingBytes	256 MiB (0 disables)
  --max-write-batches-in-flight	maxWriteBatchesInFlight	4
  --max-read-batches-in-flight	maxReadBatchesInFlight	4
  --batching-threads	batchingThreads	1

• Add SizeConverter, so --max-pending-bytes accepts a size with a binary unit suffix (512K, 10M, 1G, 2T, case-insensitive). A plain number is still read as bytes. Invalid input such as 10X or 1.5G gives a normal command-line error:

  "--max-pending-bytes": couldn't convert "10X" to a size (e.g. 512K, 10M, 1G)
  

Verification

• Added SizeConverterTest (plain bytes, each suffix, lowercase, invalid input and overflow), the first unit test in the perf module.
• :perf:test and :perf:spotlessCheck pass.
• Ran the shadow jar: --help lists the new flags, and invalid sizes are rejected as shown above. Not run against a server.