Implement infrastructure for smart contract to call back into runtime

[Robbepop]

Currently smart contracts are pretty isolated.
Wearing thick gloves one can call other remote smart contracts and even call back into the runtime, however, there is currently no support from the pDSL for doing that.

What we would like is a clean approach in calling into the runtime from the smart contract side.
This can be done by using ext_dispatch_call from the SRML contracts runtime module. It's documentation is currently unclear (at least to me) - further investigation needed.

Also we maybe need some infrastructure around it.

• Find out how ext_dispatch_call is supposed to be called from a smart contract.
• Implement ext_dispatch_call rudimentary in pdsl_core
• Implement ext_dispatch_call in pdsl_model
• Optionally provide additional helper framework in order to improve the user experience while using ext_dispatch_call as good as possible for the current state of pDSL.

[Robbepop]

For an MVP we want pdsl_core to depend on Substrate's node-runtime crate to have a common set of types to operate on. This also includes types such as Call which we can then use to make simple dispatches in pDSL to its underlying runtime.