[Rust] Add persistent stream proto

[elenagaljak-db]

What changes are proposed in this pull request?

This PR adds the in-development protobuf contract used by persistent Zerobus ingestion streams. It contains only the wire-level API.

The contract adds:

• PersistentStream, a bidirectional RPC whose first message creates a durable stream or resumes one by stream_id.
• ResumeIngestStreamRequest, including the protobuf descriptor and required record_type used to validate a resumed stream against its original configuration.
• ResumeIngestStreamResponse.last_committed_offset, the server watermark from which the client continues at last_committed_offset + 1.
• Persistent envelopes for create, resume, ingestion, acknowledgements, and server close signals.
• The server-side RetireStream contract; a public Rust retirement API is intentionally deferred.

The shapes and field numbers match the current Shinkansen server contract. The PR also records this API-contract addition in rust/NEXT_CHANGELOG.md.

Stack

1. [Rust] Add persistent stream proto #761 — protobuf contract
2. [Rust] Add persistent gRPC transport #762 — gRPC transport and recovery engine
3. [Rust] Expose persistent stream API #763 — public Rust API
4. [Rust] Test persistent stream resume #764 — stateful mock and integration tests
5. [Rust] Document persistent streams #765 — documentation and runnable example

How is this tested?

The protobuf is compiled by the Rust SDK build. The complete create, ingest, flush, and resume flow is exercised in #764.