Add csharp.supressBuildAssetsNotification setting to allow the 'Required assets to build and debug' notifications to be supressed

[seesharprun]

I regularly teach classes around using the .NET CLI and this setting is pretty obtrusive.

I have students run dotnet build and dotnet run manually so I reguarly get asked if there is a way to suppress this notification.

I'll share an example:

• I'm teaching a class on how to create a new project using the CLI and build it
• Open an empty folder
• Run dotnet new console
• Open the folder in Visual Studio Code
• The prompt will show up. We are not ready to do a build yet. We can dismiss the notification for this project, but not for future folders

This PR proposes to fix this issue by adding a new setting.

[dnfclas]

All CLA requirements met.

[codecov]

Codecov Report

Merging #3538 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3538   +/-   ##
======================================
  Coverage    89.8%   89.8%           
======================================
  Files          59      59           
  Lines        1589    1589           
  Branches       89      89           
======================================
  Hits         1427    1427           
- Misses        150     151    +1     
+ Partials       12      11    -1

Flag	Coverage Δ
#integration	?
#unit	89.8% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/packageManager/FileDownloader.ts	91.3% <0%> (ø)	⬆️

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[JoeRobich]

Thanks @seesharprun! Lots of people will be happy to have this setting. =)