Add OpenCensus propagators as allowed in OTel repositories

[dashpole]

Part of #1175

Addresses #3015 (comment). We don't currently list OpenCensus propagators anywhere in our specification, even though they have been implemented (e.g. go's BinaryFormat propagator).

This does not require implementing these propagators, but adds them to the list of propagators allowed in core repositories. My primary motivation is to document them somewhere in the specification for reference.

cc @jsuereth @bogdandrutu @jack-berg @reyang

[dashpole]

cc @carlosalberto as well

I stumbled on #437. It looks like the w3c spec is still a draft. I would like to propose that we allow re-adding the binary propagator purely as a tool to facilitate migrations from OpenCensus, without tackling the introduction of a standardized w3c binary propagator.

[carlosalberto]

Correct, we will need to allow binary propagation. I think it's totally fine to mention we only support binary for OC for now.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.

[carlosalberto]

Approving but we should do a follow up (requirement?) issue to clarify the binary propagation in the related section (for which an OC binary propagator in an additional language should be implemented, as a way to exercise it, etc)

[carlosalberto]

cc @bogdandrutu