Unify buf and prost protobuf build systems

[avahowell]

Currently, we have two independent systems for building the penumbra protobufs:

• buf- Buf provides a standalone protobuf build system and registry, and can autogenerate code (such as for the relayer).
• prost- Rust protobuf implementation, which is what links all of our protobufs to our crates internally and generates the rust code we use internally.

Until #1860, these two systems functioned identically. However, we now want buf to include upstream libraries instead of vendoring them, to avoid causing namespace conflicts in our consumers (such as the relayer). #1860 accomplishes that. However, now the prost side of the build system is building from different dependencies than the buf side. This happens to be okay now, but in the future this will likely cause issues.

To fix this, we should:

• Change our prost build tooling to look in the buf module cache for dependencies (see https://docs.buf.build/bsr/overview#module-cache)

[conorsch]

@avahowell Optimistically adding this to testnet 43, given the priority of the relayer work you've been doing. If you disagree, please update the milestone!

[conorsch]

Work is ongoing, untangling the protobuf build story. @avahowell reports she's going strong on this front, but it'll slip to next testnet; unlikely to be merged by EOW.

[conorsch]

For sanity's sake, let's deprioritize this ticket, and get @avahowell back into Rustlang for a bit. =)

[conorsch]

Cross-linking related work for visibility:

• Mess around with proto files in a probably doomed attempt to resolve relayer issues #2020
• proto: try buf gen #2035

[hdevalence]

Closing in favor of #2184