Do not block on sync controller start and engage existing clusters

[embik]

Summary

The migration to multicluster-runtime (#72) left us with a big regression, only a single PublishedResource's objects were actively synced.

Turns out, we are blocking the reconcile loop for the sync manager on the first actual sync controller we start, so no further sync controllers could be started. This PR addresses that problem.

In addition, we found a design flaw in the sync manager: Since we start controllers dynamically, an Engage call by a provider might not happen when a dynamic (sync) controller is already running. As such, we retrofitted dynamic engaging of existing clusters on new sync controllers to make sure we bring them up to par with any existing controllers.

What Type of PR Is This?

/kind bug
/kind regression

Related Issue(s)

Fixes #99

Release Notes

Fix api-syncagent not reconciling objects for more than one `PublishedResource`

[kcp-ci-bot]

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[embik]

/retest

looks like a flake.