[Rust] Split sdk/src/lib.rs into per-concern modules

[teodordelibasic-db]

What changes are proposed in this pull request?

sdk/src/lib.rs had grown to roughly 1800 lines, holding the crate root,
the ZerobusSdk client, and the entire ZerobusStream implementation in one
file. This change breaks it apart into focused modules along two axes: the SDK
client versus the stream, and within the stream, one file per concern. There
is no public API change.

lib.rs now keeps only the crate root: module declarations, the public
pub use re-exports, and the small shared types (StreamType,
TableProperties, ZerobusResult). Everything else moved out:

• sdk.rs — the ZerobusSdk client: the shared gRPC channel, TLS and proxy
  setup, and stream_builder / recreate_stream.
• stream/mod.rs — the stream parent module, organized per transport.
• stream/grpc/ — the gRPC transport, with ZerobusStream and its behavior
  split by concern.

Layout under stream/grpc/:

File	Concern	Boundary
mod.rs	ZerobusStream struct, constructor, Drop	gRPC-specific
types.rs	Internal types (IngestRequest, channel messages)	Transport-agnostic
ingest.rs	Public ingest_* methods	Transport-agnostic
acks.rs	flush, wait_for_offset, unacked queries	Transport-agnostic
close.rs	close, is_closed, task shutdown	Transport-agnostic
callback_handler.rs	User-callback dispatch task	Transport-agnostic
connection.rs	gRPC bidirectional stream setup	gRPC-specific
sender.rs	Outbound gRPC sender task	gRPC-specific
receiver.rs	Inbound gRPC receiver task	gRPC-specific
supervisor.rs	Create, spawn, recover loop	gRPC-specific

How is this tested?

• cargo build and cargo build --all-features
• cargo clippy --all-features
• cargo test --all-features
• cargo fmt --check

[zlata-stefanovic-db]

Linked the ticket for this: #34

[danilonajkov-db]

looks good, lets not delay merging this as you will always get a lot of conflicts :)

[danilonajkov-db]

we can use the type from types.rs for this

[teodordelibasic-db]

I had to resolve merge conflicts so I fixed this as well.