Tracecontext propagation should be used instead of B3

[ian-mi]

Describe the bug
The distributed tracing extension states that the traceparent and tracestate headers must be used to propagate tracing attributes.

Expected behavior
Trace context sent in W3C tracecontext format should be propagated by eventing components.

To Reproduce
Send events using ochttp tracecontext propagation:

t, _ = cloudevents.NewHTTPTransport(...)
t.Client = &http.Client{Transport: &ochttp.Transport{Propagation: &tracecontext.HTTPFormat{}}}

Events sent with this transport have disconnected traces as trace context is not propagated.

Knative release version
Release version 0.12

Additional context
May be appropriate to handle this in the cloudevents SDK instead.