feat: add packet size & packet rate

[WqyJh]

Add option to show tx/rx packet rates & avg packet size.

btm --show_packets

btm --show_packets --use-old-network-legend

[codecov]

Codecov Report

❌ Patch coverage is 8.92857% with 306 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.57%. Comparing base (b7fba3e) to head (970d814).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/canvas/widgets/network_basic.rs	0.00%	187 Missing ⚠️
src/canvas/widgets/network_graph.rs	0.00%	110 Missing ⚠️
src/canvas.rs	0.00%	5 Missing ⚠️
src/options.rs	75.00%	3 Missing ⚠️
src/collection/network/sysinfo.rs	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1938      +/-   ##
==========================================
- Coverage   53.37%   52.57%   -0.81%     
==========================================
  Files         119      119              
  Lines       16482    16769     +287     
==========================================
+ Hits         8797     8816      +19     
- Misses       7685     7953     +268     

Flag	Coverage Δ
macos-14	45.17% <8.92%> (-0.69%)	⬇️
ubuntu-latest	50.92% <8.92%> (-0.82%)	⬇️
windows-2022	44.70% <8.92%> (-0.69%)	⬇️

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[ClementTsang]

Overall this seems fine, though there's some shifting/legend size changing in the legend as the usage changes - I generally worked around this by just assigning default spacing for each chunk.

[ClementTsang]

(Also apologies for the late review)