Separated PcapLiveDevice statistics update worker under its own class.

[Dimi1010]

The PR separates the logic related to the statistics update worker into its own subclass to improve readability.

The main changes include:

• Moved PCapStats to be declared under internal namespace.
• Added helper method getStatistics to PcapHandle that calls pcap_stats on the handle internally.
• Added StatisticsUpdateWorker as a nested class under PcapLiveDevice to encapsulate a running statistics update worker thread.
• Replaced member variables related to the statistics update worker with a std::unique_ptr<StatisticsUpdateWorker> member field, representing an active worker.

[codecov]

Codecov Report

Attention: Patch coverage is 84.84848% with 10 lines in your changes missing coverage. Please review.

Project coverage is 83.08%. Comparing base (f842ccb) to head (c8bd08f).

Files with missing lines	Patch %	Lines
Pcap++/src/PcapLiveDevice.cpp	86.95%	3 Missing and 3 partials ⚠️
Pcap++/src/PcapDevice.cpp	71.42%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1785      +/-   ##
==========================================
- Coverage   83.10%   83.08%   -0.02%     
==========================================
  Files         283      283              
  Lines       48937    48971      +34     
  Branches    10567    10563       -4     
==========================================
+ Hits        40668    40690      +22     
- Misses       7113     7141      +28     
+ Partials     1156     1140      -16     

Flag	Coverage Δ
alpine320	75.05% <57.44%> (-0.04%)	⬇️
fedora42	75.16% <56.52%> (-0.05%)	⬇️
macos-13	80.58% <72.13%> (-0.03%)	⬇️
macos-14	80.58% <72.13%> (-0.03%)	⬇️
macos-15	80.56% <72.13%> (-0.03%)	⬇️
mingw32	70.66% <41.30%> (-0.13%)	⬇️
mingw64	70.60% <41.30%> (-0.15%)	⬇️
npcap	84.95% <80.39%> (-0.12%)	⬇️
rhel94	74.92% <57.44%> (-0.04%)	⬇️
ubuntu2004	58.53% <55.31%> (-0.05%)	⬇️
ubuntu2004-zstd	58.67% <55.31%> (-0.03%)	⬇️
ubuntu2204	74.84% <57.44%> (-0.07%)	⬇️
ubuntu2204-icpx	61.47% <60.00%> (-0.05%)	⬇️
ubuntu2404	75.12% <57.44%> (-0.04%)	⬇️
ubuntu2404-arm64	75.07% <57.44%> (-0.07%)	⬇️
unittest	83.08% <84.84%> (-0.02%)	⬇️
windows-2019	85.04% <80.39%> (-0.05%)	⬇️
windows-2022	85.07% <80.39%> (-0.05%)	⬇️
winpcap	85.22% <80.39%> (-0.04%)	⬇️
xdp	50.54% <0.00%> (-0.05%)	⬇️

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