Implement a Halo2Verifier async service

[dconnolly]

Similar to our other async tower-based verifier services, such as the Groth16Verifier. We should depend on the same version of the halo2 crate as we do in zebra-chain.

While we don't have batch math available yet for Halo2, the async service will decouple the verification requests from the fullfillment, allowing us to swap out the original single-verification implementation here with a more efficient batch verification implementation that is oblivious at the API level.

Testing:

Generate test vectors for halo2 proofs from zcash-hackworks; if they give trouble in our full validation pipeline, punt this full integration until we get the first NU5 block with an Action in it for testnet.

Additional context
#1951

[teor2345]

Moving this out of NU5 because it's not strictly required for activation

[dconnolly]

It's not? Otherwise halo2 verification will block everything else in the transaction verifier

[teor2345]

It's not? Otherwise halo2 verification will block everything else in the transaction verifier

We could just spawn a non-blocking tokio task, and await its completion.

[teor2345]

@dconnolly and I discussed this issue, and we decided that we should continue to use the same architecture as the other verifiers.

(Otherwise, we risk breaking other fragile async code.)

[daira]

While we don't have batch math available yet for Halo2 [...]

zcash/halo2#282 ("Document how to do simple (not aggregated) batch verification for Halo 2")

[mpguerra]

blocked on circuit from ECC