execution-core: Hide Payload and TxSkeleton structures from the public API

[moCello]

Summary

The internal Payload in both moonlight and phoenix transaction can be considered an implementation detail that doesn't need to leak to the user.

With this change execution-core needs to have a mechanism to turn the circuit-data to a proof.
This will be solved by introducing a Prover trait which can be used to implement different proof-creation mechanisms (like caching).

This will be partially fixed by #2078 which implements the changes and only stopping short of making the phoenix-payload private to not loose the previous mechanism of creating transactions.