feat(potel): Make tracing APIs use OTel in the background

[sentrivana]

WIP

• make start_span, start_transaction create OTel spans in the background via start_as_current_span
• add top level start_inactive_span that uses start_span in the background
• allow to interact with the result of start_span, start_inactive_span, start_transaction as one would with a Sentry span/transaction (i.e., find a way to keep the API the same)