Provide symbol packages (.snupkg) for Kerberos.NET and related NuGet packages

**Is your feature request related to a problem? Please describe.**
I love being able to build on top of **Kerberos.NET** by simply adding a NuGet package to my project, but if I run into a Kerberos error related to the way I'm using the library, I can't debug down into **Kerberos.NET** code to see what I'm doing wrong.

**Describe the solution you'd like**
I would love to have `.snupkg` symbol packages for **Kerberos.NET** and **Kerberos.NET.PortableDns** so that I can walk through the code in the debugger and see where I have maybe not supplied the right configuration or parameters in using the library.

**Describe alternatives you've considered**
My workaround for now has been to write a quick test application project directly in the **Kerberos.NET** source where I have cloned the repository. This works well, but takes time and could potentially go awry if my quick test application doesn't represent my main project well.

**Additional context**
Maybe debug symbols are available another way that I'm not aware of.