Implement DbProviderFactory loading for .NET Standard

[dermeister0]

https://github.com/Saritasa/SaritasaTools/blob/develop/src/Saritasa.Tools.Messages/Common/Repositories/AdoNetMessageRepository.cs#L119

dotnet/corefx#25410

[dermeister0]

Add examples of existing factories.

• System.Data.SqlClient.SqlClientFactory.Instance
• Microsoft.Data.Sqlite.SqliteFactory.Instance

[krasninja]

For 2.1?

[dermeister0]

Yes.