Halo2: Upload keys at runtime

[ElusAegis]

Create a Halo2State just like there is a mopro_core::middleware::circom::CircomState to allow to load keys on the fly.

Difficulties

Unlike Circom, the Halo2 prover code right now is baked in at compile time, so even if we were able to upload different keys, they still must be for the compiled circuit code, and can not be for a different circuit.

[ElusAegis]

Done in #214