Draft: Rewrite the Provenance service as a gRPC server in Rust, backed by Parquet

This depends on a small set of changes to Datafusion that I'll try to upstream, so Datafusion doesn't re-parse the Page Index on every query not needed anymore

All the code in rust/src/database/, other than rust/src/database/mod.rs itself, is boring boilerplate that I may try to upstream to Datafusion next.