Add WaitingPlugin support

[bsneed]

• Added ability for plugins to inform the system they need to pause event processing.
• Added WaitingPlugin conformance and auto-pause during add(plugin).
• Added analytics.pauseEventProcessing.
• Added analytics.resumeEventProcessing.
• Fixed issue where update(initial) would be called twice on destination plugins.
• Added tests.

[codecov]

Codecov Report

Attention: Patch coverage is 94.57364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 70.80%. Comparing base (e83f63e) to head (024c239).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/Segment/Waiting.swift	89.58%	5 Missing ⚠️
Sources/Segment/Settings.swift	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
+ Coverage   70.25%   70.80%   +0.54%     
==========================================
  Files          48       49       +1     
  Lines        4794     4898     +104     
==========================================
+ Hits         3368     3468     +100     
- Misses       1426     1430       +4     

Files with missing lines	Coverage Δ
Sources/Segment/Analytics.swift	84.56% <100.00%> (+0.04%)	⬆️
Sources/Segment/Plugins.swift	96.15% <100.00%> (+0.32%)	⬆️
Sources/Segment/State.swift	97.40% <100.00%> (+1.52%)	⬆️
Sources/Segment/Settings.swift	59.12% <77.77%> (+1.19%)	⬆️
Sources/Segment/Waiting.swift	89.58% <89.58%> (ø)

... and 1 file with indirect coverage changes

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