Future ChildStorage design and capabilities

[mustermeiszer]

Before submitting a PR, I thought it would be best to agree on a "spec", and more importantly if you guys even want such a core element to be externally submitted. I have thought about it for the past few days and wanted to draft my approach here and get some feedback.

Referencing:

paritytech/polkadot-sdk#278
paritytech/polkadot-sdk#363
paritytech/polkadot-sdk#368

As a disclaimer, in my "design" I am assuming that it is wanted, that the child storage is open for non-substrate data structures. If this is not wanted, closing this issue is the most straightforward.