Make UUID generation Lazy / Create PropagationContext class

[antonpirker]

We want to lazily create UUIDs to decrease the number of CPU cycles we use.

There are two important places where UUIDs are created:

1. Span/Transaction creation: https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/tracing.py#L145-L146
2. When a propagation context is created: https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/scope.py#L225-L232
3. When creating a Profile: https://github.com/getsentry/sentry-python/blob/sentry-sdk-2.0/sentry_sdk/profiler.py#L455
4. When creating a Session: https://github.com/getsentry/sentry-python/blob/sentry-sdk-2.0/sentry_sdk/session.py#L49

Solution brainstorm

1. There is a PR that already started the work: Make UUID generation lazy #2826
   This already fixes point 1. from above.
2. For point 2. we need to create a class PropagationContext and create the uuid4 only when the trace_id or the span_id is read.
3. Implement similar to 1.
4. Implement similar to 1.

[sl0thentr0py]

PR #2826