Implement SNARK pre-aggregation primitives in STM library

[jpraynaud]

Why

We need to refactor the STM library in order to make it SNARK-friendly. Following the definition of the architecture in #2763, we have defined an implementation plan and we want to address the SNARK pre-aggregation primitives part of the plan as a second step.

What

Implement the refactoring of the SNARK pre-aggregation primitives.

How

• Step 4: Implement SNARK pre-aggregation primitives (i.e. not the circuit(s) themselves)
  • Create a halo2 proof system sub-module in the proof_system module (it will embed all the halo2 circuits -non-recursive, recursive, etc.- in the future)
  • Implement the creation of the single signature artifacts using schnorr signatures
  • Adapt the key registration to support the new halo2 proof system