re_grpc_headers

Rerun gRPC header conventions.

Contains the well-known x-rerun-* header names, the RerunVersionInterceptor that stamps every outbound request with the client (or server) identity and version, the matching tower Layer helpers that wire it into a stack, and a small fork of tower-http::propagate_header used to propagate multiple Rerun headers between requests and responses.

Everything here is plain tonic/tower/http plumbing — no rerun-internal types — so it can sit on the crates/utils tier and be consumed by any crate that needs the same gRPC header behavior.