Add example for simple upgradeable contract

[cmichi]

We should add an example for a simple upgradeable contract, which uses 3-layers:

• A delegate contract which acts as a proxy and contains the address of the logic contract to call. The logic contract address can be updated only by an authorized address.
• A logic contract which contains the business logic.
• A data/model contract which exposes access to the data storage.

[HCastano]

@cmichi Hey, are you still working on this? If not, can I give it a go?

[cmichi]

@HCastano Feel free to give it a go, but the whole thing is way more complicated unfortunately, which is why I've put it on hold in April.

So the short version is that the issue needs a couple things to be implemented before it can be started. This is because the contracts pallet (and ink!) are currently lacking the means of forwarding a call. This is necessary for implementing the proxy pattern, where the proxy contract forwards the call to another contract. At the time we discussed this and came up with #739 to allow forwarding calls. So that issue has to be implemented before this one here can be started, since we can't have a contract that forwards calls otherwise.

But the story doesn't stop there, this issue #739 is blocked by paritytech/substrate#8909 unfortunately though. But that one is about to be merged.

So what you could do (if you want) is to review the Substrate PR, implement #739 as a follow-up and then implement this issue :-).