ref #8352

For more details about PropagationContext, see
https://www.notion.so/sentry/Tracing-without-performance-efab307eb7f64e71a04f09dc72722530

Building off of work in both
#8403 and
#8418, this PR adds
`PropagationContext` and uses that to always set a trace context on
outgoing error events.

Currently if there is an active span on the scope, we automatically
attach that span's trace context to all outgoing events. Now, we want to
rely on either the active span or fallback to the propagation context to
ensure that there is always a trace being generated and propagated.

Next up we'll work on updating the node/browser SDKs to update the
propagation context. For example, we should update the propagation
context for node based on the incoming sentry-trace/baggage headers.