[umbrella] Implement level-triggered architecture

[squaremo]

Implement the level-triggered architecture as explained in https://github.com/weaveworks/weave-gitops-private/blob/191-pipelines-watcher/docs/rfcs/0006-pipelines-level-triggered/README.md.

This gives a chunking of the implementation, so that each bit doesn't take too long, and some things can be done concurrently. In particular, the prototype implementation of the first issue below gives a base for reimplementing promotions, and can be extended to support remote clusters independently.

First:

• Prototype the indexing/watching behaviour needed for level-triggered promotions #194

Then, to some extent concurrently:

• Rewrite the promotion algorithm, using stubs for promotions #195
• Extend application object watching to remote clusters and arbitrary types #196
• Make promotions work "usually once" and move the handling of them to the controller #197 (needs Rewrite the promotion algorithm, using stubs for promotions #195)
• Interpret the state of a pipeline run for the UI, in the API weave-gitops-enterprise#3637 (do in tandem with UI updates)
• Check the state of an automated PR and report it in pipeline status #199 (needs Make promotions work "usually once" and move the handling of them to the controller #197)