Expose loadVerificationKey in backend_barretenberg

[signorecello]

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1. Split backend_barretenberg into prover and verifier classes #3450
   enhancement js +2
   

Problem

Sometimes you'd want to actually load an external verification_key instead of initializing one from the circuit. For example, one could load it from the output json of nargo compile --include-keys.

Reason being that it's just faster than having to compute the vk in case you already have it, or you're storing it somewhere on the browser

Happy Case

I want to be able to initialize my Backend instance with a precomputed VK. Example:

const backend = new Barretenberg(circuit, { threads: 8, importVk: true })

This would call acirLoadVerificationKey and load it from the circuit.verification_key field in the compiled circuit.

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[TomAFrench]

Closing this as the only time it would make sense to use is when you're just verifying an externally produced proof, at which point you would use @michaeljklein's work on #3450