[Prototype] Move SpanContext into Span

[carlosalberto]

Description

Prototype for moving SpanContext into Span (open-telemetry/opentelemetry-specification#1022)

For simplicity purposes, only updated:

• API
• SDK
• The Jaeger exporter

I'd like to get the Maintainers feedback - from my side here are my points:

• This prototype required changes that have been only very recently merged to the Specification (such as Define Propagation-only Span to simplify active Span logic in Context. opentelemetry-specification#994). Although we have prototypes for the aforementioned changes, we actually won't know how good it all these recent changes will play along with this specific change, and there's the general possibility that this may become a can of warms.
• The changes, from instrumentation code perspective, do not really represent a change, as mostly this has to be updated in the API/SDK/Propagators (no instrumentation, based on my digging, would need to be greatly updated).
• Now Link will either reference Span (as shown in the prototype) or replicate the Trace/Span/ ids and flag/state. Not a pretty change, but it will work.
• The change itself is neither a big issue nor a big improvement for Python overall. It won't break much existing user's code, but it doesn't bring anything useful to the table IMHO.

cc @lzchen @codeboten in case I may have overseen either good or bad impacts of this change ;)

[codeboten]

Looks like the spec change was closed, are you ok with closing this PR @carlosalberto?

[carlosalberto]

Closing this as the related PR in the Specification as closed as well.