fix: exclude native gRPC assemblies from package content (#33)

Native gRPC assemblies (grpc_csharp_ext.*.dll, libgrpc_csharp_ext.*) were being
packaged as contentFiles, causing them to appear as linked items in Solution
Explorer when consuming the package.

These files are still needed at runtime but should flow through transitively
from the Grpc.Core dependency rather than being bundled in this package.

chore: specify project path for restore and build commands

docs: update contributors

chore: add nuget trusted publishing