Remove `protoc` requirement

Trying to add lance as a dependency currently results in a build error:

error: failed to run custom build command for `lance-encoding v0.18.2`
Caused by:
  process didn't exit successfully: `lance-encoding-b1cb34df4c6a6aa9/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=protos

  --- stderr
  Error: Custom { kind: NotFound, error: "Could not find `protoc`. If `protoc` is installed, try setting the `PROTOC` environment variable to the path of the `protoc` binary. To install it on Debian, run `apt-get install protobuf-compiler`. It is also available at https://github.com/protocolbuffers/protobuf/releases  For more information: https://docs.rs/prost-build/#sourcing-protoc" }

You should instead check in the generated code, and include it in the crate. The build.rs should do nothing (or ideally, not exist).

I would consider this issue done when cargo build -p lance works WITHOUT having protoc already on PATH

Start in the lance-encoding build.rs and inspect the protos input and current generated-code flow. Run cargo build -p lance in an environment without protoc, then verify the build succeeds without that executable available.