[FEAT] Emit application events on wadm topic

[lachieh]

It would be good to have some more events to be able to subscribe to for outside systems to understand the state of an application. These are a few events that I know I'd be interested in using but this shouldn't be considered exhaustive.

• Application status change (+ new status message)
• Attempting change (scaling, deploying link, putting config)
• Change successful/failure
• App manifest deployed/undeployed
• Health check (?) passed/failed/constant

[protochron]

I'd also add manifest deploy/undeploy events to this list. You can kind of get at them from the wadm.notify subject, but they're very much representations of wadm's internal state since they tell you things like when scalers are being created or deleted.

My ideal scenario would be that we either refactor how the wadm.status stream works or add a new stream where that each event type is actually reflected in the subject (ex. wadm.status.default.app.manifest_deploy, wadm.status.default.status_change. That way external consumers can subscribe to only the events they're interested in using subject filters instead of parsing each message to determine the type and discarding anything that doesn't match