Make it impossible to track outcomes for items that remain in the envelope

[jjbayer]

Before #1877, we reported rate limiting outcomes for replay items that remained in the envelope. This was possible because track_outcome uses the output of EnvelopeLimiter::enforce, but that function did not actually remove replays from the envelope:

relay/relay-server/src/utils/rate_limits.rs

Lines 408 to 410 in 82af392

	let (enforcement, rate_limits) = self.execute(&summary, scoping)?;
	envelope.retain_items(|item| self.retain_item(item, &enforcement));
	Ok((enforcement, rate_limits))

Not sure what's the exact solution here, but generally speaking, any function that creates an outcome should consume the item that has been dropped.

Tasks

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1. Refactor EnvelopeContext: ref(server): Move Envelope into EnvelopeContext #1936
2. ref(server): Track outcomes in retain_items #1944
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3. Wontfix: ref(quotas): Refactor outcomes for rate limiting #1940
4. Make test work without running System
5. Wontfix: Make inner envelope private -> Won't work, because we should not update the envelope summary after the event has been extracted.

[jjbayer]

Outcomes can now be produced right where the envelope items are dropped, but the initial goal of simplifying outcome reporting in rate limits has not been achieved.

Let's take a more top-level look at how to restructure Relay's data flow and the envelope data type (hierarchical containers instead of flat list of items?) instead.