feat: resend events with exponential backoff and wait for ACK

[chetan-rns]

• Added a new event type ACK that is sent by the principal/agent when an event is processed successfully.
• EventWriter keeps track of the latest events and sends them on the gRPC stream. Both the agent and the principal will add events to the EventWriter instead of directly sending them to the gRPC stream. The EventWriter will send the events with exponential backoff until we receive an ACK from the other side. It continuously loops over the waiting events and sends them to the other side with minimal delay. The goal was to propagate the events as soon as possible with minimal delay and without blocking the other events.
• Added UID (resourceID) and resourceVersion (eventID) to the cloudevent as extensions. We do this to map an event to a particular state of a resource. For example, when we receive an ACK for an event we want to know if the ACK is for the latest version of a resource.

Note: I think there are multiple ways of designing this and I'm happy to update the approach based on the feedback.

Other choices that were considered:

1. We send events on a stream and immediately wait for an ACK. Use apimachinery's wait.ExponentialBackOff to resend events until we receive an ACK. However, the events from other resources will be blocked until the current event is processed.
2. We can improvise the above approach by running the wait.ExponentialBackOff in a separate goroutine for each app. However, each goroutine is shortlived (they only exist until we receive an ACK) and there might be a scenario where the runtime spends more time context switching if there are a large number of apps.
3. We could potentially solve this using a pool of goroutines (like a boss-worker pattern). But I still went with the current approach (continuously loop and resend) since there is minimal delay and we don't have to wait for the ACK sequentially.

Fixes: https://github.com/argoproj-labs/argocd-agent/issues/117

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.24460% with 155 lines in your changes missing coverage. Please review.

Project coverage is 47.21%. Comparing base (72eaea7) to head (2525ac2).

Files with missing lines	Patch %	Lines
internal/event/event.go	58.49%	62 Missing and 4 partials :warning:
principal/event.go	26.00%	36 Missing and 1 partial :warning:
agent/connection.go	0.00%	24 Missing :warning:
agent/inbound.go	23.80%	14 Missing and 2 partials :warning:
principal/apis/eventstream/eventstream.go	50.00%	9 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #225 +/- ## ========================================== + Coverage 46.82% 47.21% +0.38% ========================================== Files 57 57 Lines 4952 5151 +199 ========================================== + Hits 2319 2432 +113 - Misses 2452 2529 +77 - Partials 181 190 +9 ```

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[jannfis]

Great stuff, @chetan-rns!

I'm starting a review cycle. Please bear with me, as my reviews are dripping in :)