Add TracingConnection decorator for creating Open Telemetry spans

The component services for bigquerycontrol and bigquery_storage provide their own `*TracingConnection` classes, but those only create spans for their individual RPCs that we may call and thus there's no overarching parent span for methods that call a combination of RPCs. Adding a `TracingConnection` decorator for the `bigquery_unified::Connection` would provide a place to create such a parent span.

There may be some complications if both the `bigquery_unified::TracingConnection` and the component `TracingConnection` create spans, so it needs some investigation. If it turns out only one of the decorators can create the span, then we may need a more creative solution.