Async package

[ILMTitan]

Make GoogleCloudExtensionPackage an async package. Fix warnings flagged by Microsoft.VisualStudio.Threading.Analyzers. Fix a couple of deadlocks caused by writing to the output window on the wrong thread. Fixes #1008 Fixes #916

[iantalarico]

I started looking through this and there seems to be a lot of changes going on. I know it's a lot of clean up but any way this can be split up?

[ILMTitan]

Unfortunately, the only things that can really be separated are some of the more trivial changes such as changing MethodName -> MethodNameAsync, or adding ThreadHelper.ThrowIfNotOnUIThread().

Most of the big changes need to go together or else tests fail.

[codecov[bot]]

Codecov Report

Merging #1074 into master will decrease coverage by 0.41%. The diff coverage is 42.77%.

@@            Coverage Diff             @@
##           master    #1074      +/-   ##
==========================================
- Coverage    49.8%   49.39%   -0.42%     
==========================================
  Files         479      480       +1     
  Lines       11731    11943     +212     
==========================================
+ Hits         5843     5899      +56     
- Misses       5888     6044     +156

Impacted Files	Coverage Δ
...AttachDebuggerDialog/AttachDebuggerFirewallPort.cs	76.78% <ø> (ø)	:arrow_up:
.../CloudExplorer/CloudExplorerToolWindowControl.xaml	0% <ø> (ø)	:arrow_up:
...CloudSourceRepositories/CsrCloneWindowContent.xaml	0% <ø> (ø)	:arrow_up:
...udExtension/ProgressDialog/ProgressDialogWindow.cs	0% <ø> (ø)	:arrow_up:
...rrorReporting/ErrorReportingToolWindowControl.xaml	0% <ø> (ø)	:arrow_up:
...porting/ErrorReportingDetailToolWindowControl.xaml	0% <ø> (ø)	:arrow_up:
...itTrafficManagement/SplitTrafficWindowContent.xaml	0% <ø> (ø)	:arrow_up:
...ebuggerDialog/EnablePowerShellPortStepViewModel.cs	0% <0%> (ø)	:arrow_up:
...oudExtension/GoogleCloudExtension/Git/GitCommit.cs	0% <0%> (ø)	:arrow_up:
...n/GoogleCloudExtension/ApiManagement/ApiManager.cs	13.41% <0%> (ø)	:arrow_up:
... and 84 more

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[ILMTitan]

PTAL