zcash_client_backend: Add gRPC bindings behind feature flag

[str4d]

The lightwalletd service file is sourced from zcash/lightwalletd@ad5ecda5fbb5e12799f926b8cd785d54fdd250c8

Closes zcash/librustzcash#585.

[str4d]

tests & Clippy lints appear to be failing.

That's expected; this PR requires a newer Rust than our MSRV, due to required dependencies. We can merge this when we bump our MSRV, which will be after #646.

Is the .proto file here the canonical source of truth going forward? How do we ensure it doesn't get out of sync with lightwalletd?

Protobuf files, and gRPC on top of them, are both designed to be forward-compatible, so there's no need to synchronise updates on the client and server side. I would consider this repo to be the canonical source for compact-formats.proto (because it defines common data structures) and lightwalletd to be the canonical source for service.proto (because it defines the services that lightwalletd exposes), but it doesn't actually matter; lightwalletd can be updated with changes to its gRPC interface, and as long as those changes are made correctly within the gRPC framework then they won't break existing clients, which can then update to the new service.proto whenever they need the updated features.

[zancas]

I'll prioritize checking zingolib compatibility with this. Thanks!

[str4d]

This is now based on #691 and uses tonic.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (prost@379b703). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

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[str4d]

Clippy lints fixed in https://github.com/zcash/librustzcash/pull/691/commits/587e1fa60b4d408cfb3762fced23e8b620a71532.