[Rust] Expose persistent stream API

[elenagaljak-db]

What changes are proposed in this pull request?

This PR exposes the feature-gated Rust API for creating and resuming persistent streams on top of #762.

It adds:

• ZerobusSdk::persistent_stream_builder().
• PersistentStreamBuilder with the existing table, authentication, format, recovery, timeout, backpressure, and callback configuration.
• build() for a new durable stream and resume(stream_id) for reconnecting.
• PersistentStream, exposing stream_id, the resume watermark, ingestion, batch ingestion, flush, offset waiting, close, and unacknowledged-record retrieval.
• Shared builder preparation so ephemeral and persistent streams resolve identical gRPC configuration.

JSON and protobuf are supported; Arrow Flight and a public retirement API are intentionally deferred. Documentation follows the pipelined loop-then-flush() pattern. The new public feature is recorded 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?

• cargo check -p databricks-zerobus-ingest-sdk --features eos
• cargo test -p databricks-zerobus-ingest-sdk --features eos --lib — 156 tests passed
• cargo fmt --all -- --check
• Public create/resume behavior is exercised in [Rust] Test persistent stream resume #764.