add proc_macro to derive DbRecord instances

[Roland Kuhn]

Should not be clever, just collect all currently distributed information within the declaration of the struct, to avoid mistakes.

[Mario Camou]

Should have the ability to override default SQL types (e.g. what size varchar is this string supposed to be). Perhaps via field annotations?

[Oliver Wangler]

maybe consider https://github.com/Actyx/event-projector-to-sql/issues/50