fix(envelope): Detect attachments that create events

[jan-auer]

In addition to crash reports, the __sentry-event and __sentry-breadcrumb* attachments also create event payloads, and should therefore emit outcomes in Relay. This patch ensure that Item::creates_event correctly detects these, which is used in EventManager.

Changelog (Internal): Emit outcomes for event payloads submitted in attachment files.

[untitaker]

lgtm can you update the code comment at L 446 though

[untitaker]

Not sure about this one

[jan-auer]

Breadcrumbs alone can also create an event payload in this code path:

relay/relay-server/src/actors/events.rs

Lines 427 to 432 in b64cfb2

	return Ok(Self::event_from_attachments(
	&self.config,
	attachment_item,
	breadcrumbs_item1,
	breadcrumbs_item2,
	)?);