Move B3 and Jaeger trace propagators

[breedx-splk]

We think it makes short-term sense to continue treating the extensions/trace-propagators like a "contrib" space to contain 3rd-party/contributed propagators (see the conversation in #2207). With that, it makes less sense to leave the B3 and Jaeger propagators there, because they are first-party, supported propagators.

Let's move those two propagators into the opentelemetry-api-trace module (api/trace) and have them live in the io.opentelemetry.api.trace.propagation package as peers of the W3CTraceContextPropagator.

[jkwatson]

I'd like some resolution on open-telemetry/oteps#142 before making this decision.

[bogdandrutu]

I think it is too late for this :) Long live jaeger and zipkin in the extensions/trace-propagators