Added preliminary support for DSN. Refs #4396

@markstory @AD7six thoughts on moving this into a Trait within the Core namespace and also adding it to Cache and Log?

- [x] Cache
- [x] Database
- [x] Email
- [x] Log

Do we ever need DSN parsing in instance config classes? I know I originally proposed putting this in the StaticConfigTrait, but it _might_ be more flexible as a separate trait.