test: Move Protobuf conformance test

[slowli]

What ❔

Moves the Protobuf conformance test binary to an example in the zksync_protobuf crate.

Why ❔

zksync_protobuf relies on some "heavy" dependencies (e.g., Tokio, tracing-subscriber) just to implement the conformance test.

[slowli]

There are a couple of other options which would have the same effect:

• Making conformance_test an example in the zksync_protobuf crate instead of a binary target. This would allow to have additional deps it brings as dev dependencies (i.e., they would not be included in the dependency graph for crates depending on zksync_protobuf).
• Gating conformance_test binary behind an opt-in feature. This looks kind of awkward since IMO conformance_test is not an integral part of zksync_protobuf (it's, well, a test).

[pompon0]

I'd prefer to put it to examples then (assuming this doesn't have any nasty side-effects) as this is a test of this specific crate. IMO moving it to a crate with random stuff detaches it too much from the code under test.