Add task that generates .props file with SourceLink info for a source package

To provide SourceLink information for a source package to the project that references such package one can write a task that generates .props file below and include it in the `build` directory of the source package (the below example is from [Microsoft.CodeAnalysis.PooledObjects](https://dotnet.myget.org/feed/roslyn/package/nuget/Microsoft.CodeAnalysis.PooledObjects) package):

```xml



```

The target in Roslyn repository that generates the above props is:

```xml

<_RelativePath>$([MSBuild]::MakeRelative($(RepoRoot), $(MSBuildProjectDirectory)))


<_RawUrl>$(RepositoryRawUrl)/$(GitHeadSha)/$(_RelativePath.Replace('\', '/'))/*

<_Content>

]]>








```

Generalize this target to a build task and expose it from Microsoft.SourceLink.Common package.

To complete the end-to-end scenario we would want to also have NuGet Pack support for source packages. Then we could integrate the above task into the build system automatically when Microsoft.SourceLink.* package is referenced.