Include SourceLink PDB In NuGet Package To Support Debugging

Adding SourceLink information to the Portable PDB distributed with the software will allow package consumers to easily step into the Couchbase SDK when debugging their projects.  This may help with issue diagnosis and provide better details in bug reports.

https://github.com/ctaggart/SourceLink

It does require that the consumer to use VS2017, but only if they're stepping into the code.