*: replace rust-protobuf with prost

It's time to switch from rust-protobuf to Prost. There are a few factors
in this decision:

  * The maintainer of rust-protobuf has announced that the project is
    essentially unmaintained for the time being [0].

  * Prost is the more popular Protobuf library across the ecosystem. In
    particular, the Tonic GRPC library uses Prost, and the Tokio tracing
    console uses Prost. These are both extremely important for
    Materialize Platform, which we expect to be built on top of Tonic
    and making heavy use of distributed tracing.

There have been been two major blockers to adopting Prost:

  1. Prost lacked dynamic message support. That was miraculously fixed
     last month with the release of a fantastic prost-reflect crate [1].

  2. Prost embeds `protoc` in its Git repository (!). See
     tokio-rs/prost#575. That is a disaster for supply chain security.
     This commit patches in a Materialize-specific fork of Prost that
     compiles `protoc` from source rather than using upstream's embedded
     binary blobs.

This commit excludes one crucial piece: converting the dynamic Protobuf
decoding in the `interchange` and `testdrive` crates from rust-protobuf
to prost-reflect. I'll do that migration in a future commit to avoid
inflating the size of this PR unnecessarily.

[0]: stepancheg/rust-protobuf#583 (comment)
[1]: https://www.reddit.com/r/rust/comments/rsg6gx/announcing_prostreflect_a_crate_for_protobuf/

Cargo.toml

@@ -82,3 +82,6 @@ debug = 1
 headers = { git = "https://github.com/MaterializeInc/headers.git" }
 # Until https://github.com/jorgecarleitao/parquet-format-rs/pull/2 is merged and released
 parquet-format-async-temp = { git = "https://github.com/MaterializeInc/parquet-format-rs", branch = "main" }
+# Waiting on https://github.com/tokio-rs/prost/pull/576.
+prost = { git = "https://github.com/MaterializeInc/prost.git" }
+prost-types = { git = "https://github.com/MaterializeInc/prost.git" }