Adding watch functionality to AppSet

[reggie-k]

Summary

Add watch functionality for AppSet, similar to watch for Application, to enable AppSet support in the UI, like this:

https://github.com/argoproj/argo-cd/blob/3557ec57dc87b2570cf9beaaf8b07bb419883a45/server/application/application.go#L1060

The watch function for AppSets should be in this file: https://github.com/argoproj/argo-cd/blob/master/server/applicationset/applicationset.go

The API stream endpoint for apps is here: https://github.com/argoproj/argo-cd/blob/3557ec57dc87b2570cf9beaaf8b07bb419883a45/server/application/application.proto#L322

Here is where the API stream endpoint for AppSet should probably be:

https://github.com/argoproj/argo-cd/blob/3557ec57dc87b2570cf9beaaf8b07bb419883a45/server/applicationset/applicationset.proto

Motivation

To invoke stream API for AppSets, which would be used for the UI.

Proposal

This would need changes in the API and maybe the cli as well. The ui changes are addressed in a separate PR. This is the URL that is invoked from the UI for Apications:

https://github.com/argoproj/argo-cd/blob/3557ec57dc87b2570cf9beaaf8b07bb419883a45/ui/src/app/shared/services/applications-service.ts#L183

A similar URL should exist on the API for stream/applicationsets

[reggie-k]

For Application, there is also a watch for the ResourceTree implemented - I wonder whether we need it the same way for AppSet in the backend ?

[reggie-k]

@crenshaw-dev Is anyone working on this issue?

[reggie-k]

WIP by @alexymantha