Measure pacing and rtt spread

[Alan-Jowett]

This pull request adds support for tracking and reporting inter-packet pacing statistics in addition to the existing round-trip time (RTT) metrics in the client. The changes include collecting per-worker pacing samples, merging them, and outputting pacing percentiles alongside RTT percentiles in both console and JSON output.

Pacing statistics collection and reporting:

• Added a per-worker TDigest for collecting inter-packet pacing intervals (in milliseconds), along with a timestamp for calculating intervals in client_worker_context and global pacing digests (g_worker_pacing_tdigests, g_overall_pacing_tdigest).
• Implemented a new post_pacing function to record pacing samples and rotate the per-worker pacing digest when a threshold is reached.
• Updated the worker thread to compute and record pacing intervals for each packet sent, posting pacing digests at the end of the worker's run. [1] [2]
• Modified the digest merge logic to include merging of pacing digests into the overall pacing digest.

Output enhancements:

• Updated console output to print pacing percentiles (every 10% and high percentiles) alongside RTT percentiles.
• Extended JSON output to include pacing percentiles (every 10% and high percentiles) in addition to RTT percentiles.