Add experimental async transport

[srothh]

Includes:

• this PR: new shared base class for common sync/async logic
• Add transport worker factory and abstract base class for background worker #4580
• Add async task background worker #4591
• Add async transport #4614
• Integrate async transport with SDK #4615
• fix(asyncio integration): Filter SDK internal tasks from span creation #4700

Fixes GH-4568