refactor(sql): Allow multiple drivers at the same time (#1838)

* refactor(sql): Allow multiple drivers at the same time

* fmt

* remove default feature comment [skip ci]

* what was that doing there [skip ci]

* disable public methods for now

.changes/feat-multiple-sql-backends.md

@@ -0,0 +1,5 @@
+---
+sql: patch
+---
+
+It is now possible to enable multiple SQL backends at the same time. There will be no compile error anymore if no backends are enabled!