SourceLink support

[jonpryor]

csc /sourcelink is a way for .pdb files to contain url references to source code, not local filesystem paths.

Meaning Mono.Android.pdb could contain links into this git repo, instead of (meaningless) filesystem paths on the machine that build Xamarin.Android.

Supporting SourceLink should make the debugging experience nicer.

See also:

• SourceLink specification: https://github.com/dotnet/sourcelink
• Mono PR which adds SourceLink support: [bcl] Embed Source Link information mono/mono#12403