Migrate from protobuf to prost

[remoun]

protobuf v3 changes the API significantly (stepancheg/rust-protobuf#518, stepancheg/rust-protobuf#521), so updating to the latest version would involve a lot of changes.

Since we have to use prost in enclaves (because protobuf does not support no_std), we might as well switch everything to prost.

[remoun]

I started hacking on this, but I don't have the bandwidth to see it through. WIP at https://github.com/mobilecoinfoundation/mobilecoin/tree/replace-protobuf

I'm thinking we'll work on this in a feature branch, accepting that it won't build until we've finished migrating all the protobuf references, then PR that into main.