Remove NetworkTables Singleton

[Gold872]

• Pass in an NT instance to every widget requiring Network Tables data
• Improve test coverage

[codecov]

Codecov Report

Attention: Patch coverage is 74.35233% with 198 lines in your changes missing coverage. Please review.

Project coverage is 61.53%. Comparing base (beb5b24) to head (c598093).

Files	Patch %	Lines
lib/pages/dashboard_page.dart	48.19%	86 Missing ⚠️
lib/services/shuffleboard_nt_listener.dart	43.13%	29 Missing ⚠️
lib/services/nt_connection.dart	63.15%	14 Missing ⚠️
lib/widgets/tab_grid.dart	83.90%	14 Missing ⚠️
lib/main.dart	30.76%	9 Missing ⚠️
...b/widgets/network_tree/networktables_tree_row.dart	50.00%	7 Missing ⚠️
...aggable_containers/draggable_widget_container.dart	57.14%	6 Missing ⚠️
...e_containers/models/nt_widget_container_model.dart	50.00%	5 Missing ⚠️
lib/services/nt4_client.dart	73.33%	4 Missing ⚠️
...able_containers/models/widget_container_model.dart	60.00%	4 Missing ⚠️
... and 10 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #42       +/-   ##
===========================================
+ Coverage   45.99%   61.53%   +15.54%     
===========================================
  Files          74       75        +1     
  Lines        7595     7774      +179     
===========================================
+ Hits         3493     4784     +1291     
+ Misses       4102     2990     -1112     

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