Add Baggage Support

The Dapr runtime has recently [added support for baggage propagation](https://github.com/dapr/dapr/pull/8649). [Here is the docs PR for reference as well](https://github.com/dapr/docs/pull/4609). Baggage is a mechanism to propagate key-value pairs across service boundaries, which is useful for passing contextual information through distributed traces.

We should add a new function that users can use to add baggage propagation support, working for **both** gRPC and HTTP. This would enable users to provide baggage without having to import OpenTelemetry themselves.

consumer code _example_:
```
ctx = client.WithBaggage(ctx, map[string]string{
"user-id": "12345",
"tenant": "acme",
})

client.InvokeMethod(ctx, ...)
```

We should have tests for both gRPC + HTTP for this functionality.

Start by locating the client invocation paths for gRPC and HTTP and reviewing how context propagation is currently tested. Add the public baggage helper described in the issue, then add coverage for baggage propagation through both transports; done means users can pass the example map without importing OpenTelemetry.

Written by the indexing model from the issue text.