ADO.NET Refactoring

[JorgeCandeias]

This is a first pass at refactoring the ADO.NET provider projects.

This pass focuses on freeing the code from the compiler directives in linked files, while moving common code to a core project.

This core project is still tightly coupled with the other ADO.NET projects as of now.

There is no effort yet to integrate with DI, that's left for a second pass.

While this PR is very intrusive, it is not meant to have any breaking changes.

Microsoft Reviewers: Open in CodeFlow