ref(cache): Move buffering of pending envelope to ProjectCache

[olksdr]

Take two as an attempt to fix the problem we had while deploying #1879 which was reverted in #1906

---

These changes moving buffering of the incoming envelopes in the ProjectCache.

Current implementation still keeps, so called queue in memory and using HashMap with a composite key QueueKey {key, sampling_key}, where sampling_key can be the same as a key if there is no sampling project identified. The values to these keys are Vec of boxed Envelope with their EnvelopeContext.

Once we get an update for project state, we check all variants of QueueKey which contains the current ProjectKey and if all the project states are cached we try to flush buffered envelopes indexed by these QeueuKey.

The envelops will be buffered if:

the project state is not fetched yet
root project is here but the sampling project state is not fetched yet
the sampling project state is here but the root project is not fetched yet
This change also removes all the buffering from the Project and reduces its responsibility. Now it just keeps its own state and configuration and the envelope handling is done outside of it.

fix https://github.com/getsentry/team-ingest/issues/76

[olksdr]

Refactor handle_processing a bit and also add more logging and docs in 2389abe

@jjbayer @jan-auer, please, take another look.

[jjbayer]

Suggested change

	/// - Optional: if dynamic sampling keys exists, the dyanmic project state must be cached and valid.
	/// - If dynamic sampling keys exists, the sampling project state must be cached and valid.

[olksdr]

also done in 56f32b8

[jjbayer]

Personally I would write this as a guard to keep the error handling close to the check, i.e.

let Some(project) = self.projects.get_mut(&project_key) else {
  // log error
  return
};

let Some(own_project_state) = ... else {
   // log error
   return
}

if let Ok(CheckedEnvelope {...}) = ... {
    ...
}

[olksdr]

Yeah, this can look better. I will change.

[olksdr]

done cdf587a