Description: More work as part of the merge project. This one is pretty straightforward - the SqlDiagnostic* classes only belonged to the netcore project, so this PR:

• Moves the files over to the common project in a Diagnostics folder
• Split the files into one class per file
• Moved classes that weren't in the SqlClient.Diagnostics namespace into that namespace
• Moved the extension methods for DiagnosticsListener class into the DiagnosticListener class
  • I can revert this if there's significant backlash to the change, but considering the code is all internal, to me, there's no real reason to keep these as extension methods.
• Also created stubs for SqlCommand and SqlConnection in common project to make it a bit easier to work on the Diagnostic files.

Testing: No functional changes to the code, CI should be able to validate.