feat(spans): enforce max segment bytes during ingestion

[lvthanh03]

• Adds the spans.buffer.enforce-segment-size option to force segment size limit of max-segment-bytes.
• When the option is enabled:
  • The Lua script checks cumulative ingested bytes of the target set to decide whether to merge or not.
  • Writes each subsegment's payloads to a separate key determined by a random salt. This is to prevent dropping overflow subsegments when we decide not to merge them into the target set. Each subsegment not being merged will become its own separate segment, and we add a new key in the queue based on the salt value above.