FerretDB .NET example

This is a fork of https://github.com/mongodb-developer/mongodb-dotnet-example, reduced to reproduce a (already fixed) problem with FerretDB.

How to run:

1. Install dotnet tool.
2. Run dotnet run mongodb://localhost:27017/.
   • To run with the strict Stable API use the --strict flag.
   • To run with authentication use credentials and authMechanism as usual: dotnet run 'mongodb://username:password@localhost:27017/?authMechanism=PLAIN'.