feat(tvix/tracing): gRPC trace context propagation

This introduces optional helper function in tvix/tracing for trace
propagation and uses these helper in the `tvix-store`.

The GRPCBlobService, GRPCDirectoryService and GRPCPathInfoService now
accept a generic client, meaning the client can be generated with either
`::new` or `::with_interceptor`.

This was tested and validated by starting a `tvix-store daemon` and
`tvix-store import`.

Change-Id: I4b194483bf09266820104b4b56e4a135dca2b77a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11863
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/docs/src/TODO.md

@@ -233,9 +233,6 @@ logs etc, but this is something requiring a lot of designing.
  - Maybe drop `--log-level` entirely, and only use `RUST_LOG` env exclusively?
    `debug`,`trace` level across all crates is a bit useless, and `RUST_LOG` can
    be much more granular…
- - gRPC trace propagation (cl/10532 + @simon)
-   We need to wire trace propagation into our gRPC clients, so if we collect
-   traces both for the client and server they will be connected.
  - Fix OTLP sending batches on shutdown.
    It seems for short-lived CLI invocations we don't end up receiving all spans.
    Ensure we flush these on ctrl-c, and regular process termination.