Simplify approach for creating dag run and task spans

[dstandish]

Replace otel spans tracking implementation with something much simpler

• rip out existing spans
• add back the dag run span
• add back a task span (this is handled in the runner)
  • the detailed task spans will be done in a followup
• rip out Trace, _TraceMeta, DebugTrace "classes" --> later
• remove the "Tracer" protocol and EmptyTrace --> later
  • we don't need to keep them for backcompat because they were never really used~
  • EmptyTrace was returned when otel not enabled. but it should be fine to lean on the normal behavior of otel to create non-recording spans when otel not enabled, and the updates to OtelTrace should accomplish this
    deprecate the OtelTrace class
  • update its methods so that it does as little as possible but doesn't break dags for any unfortunate souls that decided to use it
• re-implement the configuration bit which looks at our deprecated otel config vars and the env vars supported by otel

[xBis7]

@dstandish Great job! I left some comments but generally it looks good and I think it can be converted to an actual PR.

Additionally, I tested it manually and I ran a dag with long timeouts. I didn't see any issues.