Implement Span-First

[jamescrosswell]

Description

See developer docs:

• https://develop.sentry.dev/sdk/telemetry/spans/implementation/

TODO

This is definitely an epic, so each of the following would be their own PR:

• feat: Send transactions as Span v2 streams #4912
  • Span v2 Envelopes
  • Add traceLifeCycle option
  • Send transactions as span arrays
• Add start_span API
• Implement captureSpan
• Ensure data parity for segment spans
• Implement span buffer
• Make transactions obsolete

[linear]

NET-547