[Rust] Add stream builder API for multiplexed streams

[danilonajkov-db]

What changes are proposed in this pull request?

• Add public Rust API for multiplexed protobuf streams via:

stream_builder().compiled_proto(...).multiplexed(n).build()

• Keep existing stream_builder().build() behavior unchanged: it still returns ZerobusStream.
• Promote MultiplexedStream, MessageId, and MultiplexedStreamBuilder to the public Rust API.
• Validate multiplexed stream count and keep Arrow Flight on the existing build_arrow() path.
• Add a protobuf multiplexed example using the shared orders schema, plus Rust docs and changelog coverage.

How is this tested?

Existing unit tests + added new ones