Optimize MRC event handling on startup

[jaellio]

In response to each MRCEvent the MRC reconciler lists all MRCs in the mesh. Listing all MRCs for each event is done to achieve a level-triggered design. On startup or restart, each existing MRC triggers a create event resulting in O(NM) operations, where N is the number events and M is the number of MRCs in the mesh. This could potentially have an impact on the mesh's performance.

One approach to optimizing event handling is to use a custom informer index:

• example creation of a custom informer index: https://github.com/kubernetes-sigs/controller-runtime/blob/f46919744bee01060c9084a285e049afffd38c9d/pkg/cache/informer_cache.go#L180
• example caller to create index: https://github.com/kubernetes-sigs/controller-runtime/blob/d887b2f1c68fee51b20378ff2a0bd6a8a77674dd/pkg/client/example_test.go
• example of listing using a custom informer index: https://github.com/kubernetes-sigs/controller-runtime/blob/8ad090e0663e046c5a9dad68a572af0e30626c85/pkg/cache/internal/cache_reader.go#L126

Related discussion: https://github.com/openservicemesh/osm/pull/5201#discussion_r996326422

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