Extract design patterns from multicluster-scheduler, a.o.

[adrienjt]

Multicluster-scheduler is multicluster-controller's prime application example. It uses patterns that could be generalized and hosted here as library code for re-use by other projects. Here are some of the patterns used by multicluster-scheduler:

• push/pull observations (snapshots of objects as custom resources)
• push/pull decisions (specs of objects to be applied in another cluster)
• cross-cluster garbage collection with finalizers (foreground deletion and orphan)

Other interesting patterns:

• push/pull copies
• cross-cluster garbage collection with controllers (add background deletion support)
• ...

[adrienjt]

v0.3.0 introduced the decorator and garbage collection patterns.