feat: Add initial OpenTelemetry support

[cleptric]

This PR adds two main integration components (along with all the main supporting functions to enrich Sentry spans with OTel data):

• Span Processor
• Propagator

Main discussion: getsentry/sentry#40712

Closes #526, closes #486.

A list of open issues with the implementation:

1. Dynamic Sampling Context propagation from nested spans doesn't work properly. Main reason: no publicly exposed mechanism to get the transaction (as in, a root span) for the given nested span.
   [OTel] Dynamic Sampling Context propagation does not work for nested spans #553
2. Baggage parsing bug is not yet fixed or acknowledged: Error while parsing Baggage header with percent-encoded whitespace open-telemetry/opentelemetry-go#3601 We might want to use our vendored implementation and fix the bug there.
   [OTel] Baggage parsed incorrectly if it contains encoded whitespaces ("%20") #554
3. Associating errors with transactions is generally implemented (see otel/event_processor.go), but there's a caveat: the EventHint of the event processor has to contain a valid Context with a set OpenTelemetry span. This means that e.g. messages captured with sentry.CaptureMessage or hub.CaptureMessage won't be linked to Sentry spans at the moment.
   [OTel] Expose context-aware methods for capturing events in public API #555
4. "instrumenter" configuration in ClientOptions -- we need to confirm whether we need that.
   [OTel] Add "instrumenter" configuration option #556
5. Span attributes/description for special cases (HTTP, DB, RPC, etc.) might be incomplete.
   [OTel] Improve HTTP Span attributes in SpanProcessor #557

[cleptric]

This might be super noisy for users. I use Debug: true a lot, so maybe we could reduce the output a little bit.

[tonyo]

🚢 🚂 🚀