Threading revamp: add ThreadContext and remove Dataflow usage

[adamhathcock]

Introduces ThreadContext to centrally manage threading across host apps. Most are UI/Worker based but exceptions are noted here:
https://www.notion.so/speckle/Threading-In-Host-Apps-14ab78fc7aa6807ca052c6c0869530d2

[codecov]

Codecov Report

Attention: Patch coverage is 7.38007% with 251 lines in your changes missing coverage. Please review.

Project coverage is 9.24%. Comparing base (abc2d9b) to head (eb21a89).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
...eckle.Connectors.Common/Threading/ThreadContext.cs	0.00%	48 Missing ⚠️
...UI3/Speckle.Connectors.DUI/Bridge/BrowserBridge.cs	0.00%	35 Missing ⚠️
...e.Connectors.Common/Operations/ReceiveOperation.cs	0.00%	34 Missing ⚠️
...e.Connectors.DUI/Bindings/SendBindingUICommands.cs	0.00%	20 Missing ⚠️
....Connectors.DUI/Bridge/TopLevelExceptionHandler.cs	48.48%	16 Missing and 1 partial ⚠️
...ckle.Connectors.Common/Operations/SendOperation.cs	0.00%	16 Missing ⚠️
...onnectors.Common/Threading/DefaultThreadContext.cs	0.00%	14 Missing ⚠️
....Connectors.DUI/Bindings/IBasicConnectorBinding.cs	0.00%	12 Missing ⚠️
...le.Connectors.Common/Operations/ReceiveProgress.cs	0.00%	12 Missing ⚠️
...onnectors.DUI/Bindings/OperationProgressManager.cs	0.00%	11 Missing ⚠️
... and 6 more

Additional details and impacted files

@@           Coverage Diff            @@
##             dev    #400      +/-   ##
========================================
- Coverage   9.40%   9.24%   -0.16%     
========================================
  Files        223     227       +4     
  Lines       4222    4260      +38     
  Branches     497     533      +36     
========================================
- Hits         397     394       -3     
- Misses      3809    3850      +41     
  Partials      16      16              

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