feat: single top level meson.build, add NotificationTester and SequenceTester

[tlambert03]

the whole symlinking of MMDevice is problematic, particularly cross-platform... so this PR goes with the recommended (?) approach of having a single top level meson.build that manages all subdirectories

it does some significant cleanup of unneeded files, and also adds NotificationTester and SequenceTester (boost installed at system level, and msgpack installed via wrap)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 667 lines in your changes missing coverage. Please review.

Project coverage is 16.22%. Comparing base (418bf39) to head (0fa69f4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...Adapters/NotificationTester/NotificationTester.cpp	0.00%	409 Missing and 17 partials ⚠️
...s/DeviceAdapters/NotificationTester/ProcessModel.h	0.00%	153 Missing and 8 partials ⚠️
...eviceAdapters/NotificationTester/DelayedNotifier.h	0.00%	77 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   24.08%   16.22%   -7.86%     
==========================================
  Files          90      129      +39     
  Lines        6710    20290   +13580     
  Branches     3516     8435    +4919     
==========================================
+ Hits         1616     3292    +1676     
- Misses       3773    14625   +10852     
- Partials     1321     2373    +1052     

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