Support build.rs git-lfs fetching for proto_descriptor

[conorsch]

Is your feature request related to a problem? Please describe.
When trying to build a Rust project that uses the Penumbra crates as dependencies, cargo build fails with an error on the penumbra-proto crate, because the proto_descriptor.bin file isn't found.

Describe the solution you'd like
A clear and concise description of what you want to happen.

Describe alternatives you've considered
A workaround is to set a path dependency in the Cargo.toml file, rather than e.g. a git dependency. Provided you have the git-lfs assets checked out locally, that'll work.

Additional context
We've encountered this problem before, e.g. in Galileo penumbra-zone/galileo#29 , and our "fix" was to use path dependencies as a workaround. However there's also the dope #2201 which resolves the lfs problem specifically for the proof params.

Let's port that build.rs logic to the proto crate so that external projects can use git dependencies.

Unsure what the implications of the build.rs strategy are for future hosting on crates.io; I suggest we figure that out later.

[hdevalence]

In the meantime, we can disable LFS for the proto descriptor. (We're already checking in hex-formatted proto descriptors into the repo in the form of the generated Go code).

[hdevalence]

Closing with #3044 as "good enough" for now.