Determine number of inputs per nested tx

[dtebbs]

Currently num_inputs_per_nested_proof is hardcoded, as if it were a constant that can be set freely. The dispatcher contract also allows this to be changed (in-line with a previous approach). In fact, this is a function of the specific hash used for GGPR input hashing (if GGPR is used for nested transactions).

Suggested changes:

• Modify ZecaleDispatcher constract to be paramerized by inputs-per-nested-tx (and compute batch-size from this, rather than the other way around, since batch-size if more feely adjustable)
• Make nuum_inputs_per_nested_tx a property of the aggregator_circuit, since this determines whether GGPR is used, and with which hash.

It may be possible to further parameterize the circuit in future, and maybe support other configurations such as no GGPR (for the case of aggregating txs for a single app), but num_inputs_per_nested_tx will always be a function of the way this top-level circuit is configured.