Make UUID generation lazy

[sl0thentr0py]

same as #2131

for #2827

[sl0thentr0py]

note that we also have uuid generation on scope.propagation_context now and to make that lazy we first need to encapsulate propagation_context in a class and not just a raw dict as currently implemented.

sentry-python/sentry_sdk/scope.py

Lines 225 to 232 in 64e0ec5

	def _create_new_propagation_context(self):
	# type: () -> Dict[str, Any]
	return {
	"trace_id": uuid.uuid4().hex,
	"span_id": uuid.uuid4().hex[16:],
	"parent_span_id": None,
	"dynamic_sampling_context": None,
	}

Without also making those lazy, this will not really have a performance impact but we can merge it anyway.

[anonrig]

Is there a reason to not land this pull request right now?

[sl0thentr0py]

we can merge but without the prop context changes it won't have full impact, and that might conflict with the v2 refactor, I will let @sentrivana and @antonpirker decide

[antonpirker]

I will refactor the propagation context first and then merge this change