Create dotnet.yml

[asgerhallas]

[what-the-diff[bot]]

• Added a new file .github/workflows/dotnet.yml
• Changed the appveyor config to use build script instead of inline commands, and added github nuget feed as deploy target
• Created build script that builds HybridDb with version number from argument 1 (appveyor) or 2 (local), and pushes it to official nuget if api key is provided in argument 3 (appveyor) or 4(local).
• Fixed test for real tables so they can be used on local machine without having SQL Server installed locally first by using CREATE DATABASE IF NOT EXISTS syntax which was introduced in SqlServer 2016 SP1 CU5 https://docs.microsoft.com/en-us/sql/t-sql/statements/create-database-transact-sql?view=sqlserverver15#remarks