RD BtoXll development

Checklist for this to be merged into rd-devel:

• Dependencies merged to rd-btoxll-dev:
  • !4022 (merged)
  • !4146 (merged)
  • !4150 (merged)
  • !4157 (merged)
  • !4163 (merged)
  • !4168 (merged)
  • !4297 (merged)
  • !4376 (merged) (supersedes !4141 (closed))
  • !4356 (merged)
  • !4397 (will postpone this one to 28th March 2025)
  • !4388 (will postpone this one to 28th March 2025)
• Rebased to pick up changes in rd-devel
• Rate and bandwidth tests performed
• Rate and bandwidth results approved

Total HLT2 rate/BW for this MR (!4352 at 4a63ca8c)

Rate: 9.07 kHz (9.84 kHz, -7.8%)

Bandwidth: 0.501 GB/s (0.562 GB/s, -10.9%)

Full line-by-line comparison: hlt2_rates_comparison.md