[rust] Check in generated protobuf code and drop protoc requirement

[seokjin0414]

Re-targeted from apache/fluss-rust#662 after the Rust client moved into this repo; the old PR and apache/fluss-rust#661 are closed with pointers here. Downstream context: apache/iggy#3688.

FlussApi.proto only changes when the wire protocol does, but fluss-rust/crates/fluss/build.rs reran prost-build on every build, which makes a system protoc a requirement for every consumer of the fluss-rs crate and for the Rust CI here. Vendoring the proto into published crates (scripts/vendor-proto.sh) ships the schema, but consumers still run the codegen, so they still need protoc.

This checks the generated code in instead, following the arrow-flight layout (committed prost output + a small gen crate + regen.sh):

• crates/fluss/src/proto/fluss.rs: checked-in prost output, byte-identical to what build.rs produced (same prost-build version, same bytes config), with the ASF header and a do-not-edit notice prepended
• crates/fluss/gen: a tiny bin crate that regenerates it. prost-build is a normal dependency there, so the workspace compiles without protoc; protoc is only needed when actually running crates/fluss/regen.sh. The proto is resolved the same way build.rs did it: the vendored proto/FlussApi.proto when present, the canonical fluss-rpc/src/main/proto/FlussApi.proto otherwise
• build.rs and the [build-dependencies] on prost-build are removed; cargo package -p fluss-rs --list confirms fluss.rs ships in the crate and the nested gen crate is excluded automatically
• a new proto-up-to-date job in rust-build-and-test.yml reruns the regeneration and fails on any diff, so a change to the canonical proto cannot land without regenerating the Rust side (the workflow already triggers on fluss-rpc/src/main/proto/**). The output is deterministic: prost-build is version-locked through Cargo.lock, and generation is byte-identical across protoc 27.1, 29.3 and 35.1
• the Rust build/test and license/format workflows stop installing protoc otherwise, so CI itself now proves the protoc-free build
• DEVELOPMENT.md: protobuf is now only a prerequisite for regenerating

Verified locally with no protoc installed: cargo build, cargo clippy --all-targets --workspace -- -D warnings, cargo test with the CI excludes, cargo fmt --all -- --check, and cargo doc --workspace --no-deps --exclude fluss_python, all from fluss-rust/. Rerunning regen.sh reproduces the committed file byte-for-byte.

Once this is in a release, the protoc requirement disappears for every fluss-rs consumer; the two Fluss connectors in flight for Apache Iggy (apache/iggy#3799, apache/iggy#3782) are currently blocked on exactly that.