Draft/RFC: Adds a Service mixin that can be used to get static type safety for classes

[freider]

Very rough "implementation", just adding some static type check tests that this actually provides static type support.

This doesn't actually implement these methods, and it's a bit TBD if the actual implementation should be in in the mixin or the Obj (probably the latter?)

Preferably, the same implementation can be reused through composition in Function

This is also pending the discussion of whether to put these methods on the class directly or under a .service attribute or similar, but IMO this is the most promising and consistent way of doing it...

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Changelog

[freider]

So beautiful!