feat: Extend Frame::Padding with length

[larseggert]

This is preparation of qlog supporting the logging of runs of padding frames via payload_length, instead of each one individually.

Needs cloudflare/quiche#1745 (eventually).

[martinthomson]

Looks good.

[codecov]

Codecov Report

Attention: Patch coverage is 93.04813% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 92.98%. Comparing base (7028479) to head (aa71d49).

Files	Patch %	Lines
neqo-transport/src/frame.rs	71.42%	8 Missing ⚠️
neqo-transport/src/qlog.rs	98.02%	3 Missing ⚠️
neqo-transport/src/connection/dump.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1762   +/-   ##
=======================================
  Coverage   92.97%   92.98%           
=======================================
  Files         119      119           
  Lines       37253    37246    -7     
=======================================
- Hits        34637    34633    -4     
+ Misses       2616     2613    -3     

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[github-actions]

Benchmark results

Performance differences relative to 32ef2c3.

• coalesce_acked_from_zero 1+1 entries
  time: [196.70 ns 197.21 ns 197.77 ns]
  change: [-0.9743% -0.6245% -0.2430%] (p = 0.00 < 0.05)
  Change within noise threshold.

• coalesce_acked_from_zero 3+1 entries
  time: [237.99 ns 240.23 ns 244.49 ns]
  change: [-2.8540% -2.1868% -1.3377%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• coalesce_acked_from_zero 10+1 entries
  time: [237.43 ns 238.00 ns 238.74 ns]
  change: [-3.5165% -2.5301% -1.7019%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• coalesce_acked_from_zero 1000+1 entries
  time: [219.82 ns 220.05 ns 220.33 ns]
  change: [-2.8017% -2.0773% -1.3776%] (p = 0.00 < 0.05)
  💚 Performance has improved.

• RxStreamOrderer::inbound_frame()
  time: [120.21 ms 120.29 ms 120.37 ms]
  change: [-0.0926% +0.0084% +0.1109%] (p = 0.88 > 0.05)
  No change in performance detected.

• transfer/Run multiple transfers with varying seeds
  time: [116.88 ms 117.15 ms 117.41 ms]
  thrpt: [34.069 MiB/s 34.145 MiB/s 34.222 MiB/s]
  change:
  time: [-1.0057% -0.6906% -0.3471%] (p = 0.00 < 0.05)
  thrpt: [+0.3483% +0.6954% +1.0159%]
  Change within noise threshold.

• transfer/Run multiple transfers with the same seed
  time: [117.25 ms 117.45 ms 117.67 ms]
  thrpt: [33.993 MiB/s 34.057 MiB/s 34.116 MiB/s]
  change:
  time: [-0.9901% -0.7597% -0.5360%] (p = 0.00 < 0.05)
  thrpt: [+0.5389% +0.7655% +1.0000%]
  Change within noise threshold.

⬇️ Download logs

[KershawChang]

lgtm