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.

• Run dotnet run mongodb://localhost:27017/.
• To run with the strict Stable API use the --strict flag.
• To run with PLAIN authentication pass PLAIN to the authMechanism URI option dotnet run "mongodb://username:password@localhost:27017/?authMechanism=PLAIN".