Share preprocessed key over all proofs in batch

[dtebbs]

(Reminder for later optimization)

The aggregator holds verifiers which each preprocesses the public key (supporting the use-case where each proof in a batch uses a different vk), but its interface assumes a shared key (i.e. a batch where all proofs belong to the same "application").

• Extend the verifier_parameters structs to expose preprocess_verification_key_gadget, verification_key_precomputation_variable and online_verifier_gadget
• Use these in the aggregator_gadget to ensure the vk is processed only once