Sign the Assembly with a Strong Name

When building a strong-named assembly referencing this sink, one will get the warning below during the build.

CSC : warning CS8002: Referenced assembly 'Serilog.Sinks.Slack, Version=1.2.56.0, Culture=neutral, PublicKeyToken=null' does not have a strong name.

Signing the assembly with a strong name is a best practice, and would cause no side-effects whatsoever to anyone.