Revert PR #155 (removal of Distributed Tracing extension)

[iampluque]

As per comment on cloudevent/spec #715, distributed tracing extension should be kept in the spec.

Please revert PR #155

[jskeet]

I'll have a look when I get a chance, but it probably won't be soon as I'm on vacation for a week starting Friday. Given how extensions have been changing recently, I'm starting to think we possibly shouldn't have them in the main library at all.

[iampluque]

vacation for a week starting Friday

you are forgiven 😆

[jskeet]

Hmm. This comment in the issue is interesting:

This extension will be kept as the OpenTelemetry WG for Messaging Sementics is interested in using and potentially updating it to aligned with the outcome of their work

The "potentially updating it" suggests it's not stable right now - which is in-keeping with the status as "extension" rather than "specification-defined attribute".

I've raised #232 for a potential plan. If we go ahead with that, we would want to just create a distributed tracing extensions as a new package rather than including it in the main package then making it obsolete. Thoughts welcome!