Enable injectors and extractors bound to tracer

[bobrik]

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In jaeger-client-go propagators (injector + extractor concept) are bound to tracers, which makes it possible to wrap a default implementations. For example, we were able to sign HTTP headers with an HMAC key bound to tracer, with user working with an opaque OpenTracing interface.

It would be nice to have the same ability in rust implementation.

[sile]

Thanks for your useful suggestion. It sounds good, but probably requires breaking interface changes (maybe). Therefore, I would like to revisit this feature when actual users need it.