Support custom process groups in distributed streamer

[Broduker]

Summary

add an optional custom process group to SafetensorsStreamer and DistributedStreamer;
scope distributed rank discovery, backend checks, partitioning, barriers, and broadcasts to the provided group;
treat a provided process group as borrowed and leave its lifecycle to the caller;
add document explaining how to use it.

When no process group is provided, the existing global/host-local group selection behavior is preserved.

Fixes #165

Tests

• added unit tests for process-group-scoped rank discovery, direct group reuse, and caller ownership;
• added a distributed test that streams through a borrowed all-rank process group;
• added a distributed test where only a subset process group participates.

[noa-neria]

@Broduker thank you! Happy to review
See my suggestions regarding the design in #165