Implement seal_call_runtime

[cmichi]

With paritytech/substrate#9276 implemented, we can now introduce seal_call_runtime into ink!, providing contracts with the ability of dispatching calls into the runtime.

ToDo

• Add a new ink! API function self.env().call_runtime(…) analog to our other env() API functions (e.g. with UI tests under crates/lang/macro/tests/ui/). See the linked issue for the contracts pallet API.
• Add an example which demonstrates how to call into the runtime. It should be possible to do this analog to the rand-extension example.
• Implement a test for this new example in the ink-waterfall.
• Add a section to the ink-docs, describing how this new functionality can be used.

[athei]

You have to keep in mind that extrinsics cannot return any data (only success/fail). They communicate via events with the offchain world. So something like rand-extension isn't possible with seal_call_runtime. It is unstable for a reason. There are several open questions around it:

• Is it OK that it can't return data or should we make an effort to deliver events (asynchronously) to contracts?
• There are issues around stability. Contract APIs must be stable but extrinsics are allowed to change. Once you allow some extrinsic to be called from a contract it puts a heavy burden on the runtime to never change this API.

[Robbepop]

What is the difference from this feature to chain extensions that allow for the same functionality?

[athei]

Explained here.