Make the ProxyContainerPrototype rotatable

[everything-narrative]

TL;DR

The ContainerPrototype is as of 2.1 rotatable (yay!), the ProxyContainerPrototype is not. This is incongruent and inconvenient and seems like a slam dunk fix.

WHAT?

Basically the Proxy Container seems like it should 'just' be a container with the added feature of being a proxy inventory. It is useful in a lot of interesting ways in mods, and an absolute godsend. However it is now lacking behind the regular containers in functionality, and making a rotatable proxy container entity still needs a workaround like it did with Containers of yore.

WHY?

Would be greatly convenient for the new generation of advanced item storage mods.

[everything-narrative]

ALSO!

Almost forgot: a separate graphics set for when connected/not connected would be super cool, and an option to show a flashing icon when not connected.

[boskid]

I can see it being useful to have proxy container be directional to have it rectangular or have it pointing in a certain direction that mod would use to point at some entity, so proxy container direction is now implemented for 2.1.21. This is however a breaking change as it involves changing ProxyContainerPrototype::circuit_connector to be an array. I think there will be far less mods broken by this compared to item's fuel_categories so no breaking change announcement.

As for the other change (separate graphics based on connection state), it is out of scope.