Add NuGet configuration file

[martincostello]

• Add an explicit NuGet configuration file that only restores from NuGet.org. This also makes it easier to configure an additional NuGet package source (such as GitHub Packages) to consume pre-release packages in the samples.
• Add NuGet package source mappings.

[codecov]

Codecov Report

Merging #1305 (bc97753) into main (c782d81) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1305   +/-   ##
=======================================
  Coverage   83.28%   83.28%           
=======================================
  Files         264      264           
  Lines        6298     6298           
  Branches      996      996           
=======================================
  Hits         5245     5245           
  Misses        844      844           
  Partials      209      209           

Flag	Coverage Δ
macos	83.28% <ø> (ø)
windows	83.28% <ø> (ø)

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