[PoC] Support for Redfield

[TimothyMothra]

Changes

• Introduce build flag Redfield=True
• When detected, change version of dependency: System.Diagnostics.DiagnosticSource
• When detected set a Constant to be used in preprocessor. This is used to change the name of EventSource.

To use:

dotnet build /p:Redfield=True

To verify:

I introduced a new unit test to verify that these changes took effect.

dotnet build /p:Redfield=True ".\dotnet\BASE\Microsoft.ApplicationInsights.sln"
dotnet test ".\bin\Debug\test\Microsoft.ApplicationInsights.Tests\net5.0\Microsoft.ApplicationInsights.Tests.dll" --filter Name~VerifyRedfieldEventSourceName

Open Questions

• Do we need to run full tests against Redfield build?