Add sourcelink support for core

[ssa3512]

I am attempting to troubleshoot issues with custom telemetry correlation and it is difficult to step into the app insights library to identify what is going wrong. It would be helpful if the core library (Microsoft.ApplicationInsights) supported sourcelink. This is already supported on the netcore logging library here:

ApplicationInsights-dotnet/LOGGING/src/ILogger/ILogger.csproj

Lines 50 to 53 in 35176da

	<PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0">
	<PrivateAssets>all</PrivateAssets>
	<IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
	</PackageReference>

[TimothyMothra]

note to self: estimate the work, see if we can take this in 2.15

[TimothyMothra]

We'll need to follow this general guidance to onboard:
https://github.com/dotnet/sourcelink#using-source-link-in-net-projects

[TimothyMothra]

SourceLink has been turned on and will be available in our next 2.15 beta release. :)