Add composite ConfigStore to combine multiple ConfigStore

[pierDipi]

In some repositories, we used multiple ConfigStores to store feature flags, for example, in eventing kafka broker, we use the Eventing core feature flags store and the EKB feature flags store

Changes

• Add composite ConfigStore to combine multiple ConfigStore

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pierDipi

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• reconciler/OWNERS [pierDipi]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.73%. Comparing base (03bf3de) to head (57cad9e).
Report is 29 commits behind head on main.

Files	Patch %	Lines
reconciler/configstore.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3027      +/-   ##
==========================================
- Coverage   78.75%   78.73%   -0.02%     
==========================================
  Files         188      189       +1     
  Lines       11107     8873    -2234     
==========================================
- Hits         8747     6986    -1761     
+ Misses       2094     1621     -473     
  Partials      266      266              

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[skonto]

Do you see this being

	for _, s := range stores {
		ctx = s.ToContext(ctx)
	}

being used often in different reconcilers, besides the example in the description? Trying to understand where we can re-use it.

[pierDipi]

This can be reused everywhere we have multiple config stores (config stores being mainly configmaps), a serving example could be: config-features and config-network in the case one reconciler depends on both

	impl := <resource>reconciler.NewImpl(ctx, reconciler, func(impl *controller.Impl) controller.Options {
		return controller.Options{
			ConfigStore:       reconciler.ConfigStores{s1, s2},
                        // ...
		}
	})

and then the usage would just pull the config from the context in ReconcileKind

s1 := s1package.FromContext(ctx)
...

[skonto]

in the case one reconciler depends on both

Not sure if we have a scenario in Serving but ok I see how this can be useful.

[skonto]

/lgtm
/hold for @dprotaso if he has any objections

[pierDipi]

/unhold

@dprotaso I'm merging this as I don't see objections here

[ReToCode]

/test unit-tests

[ReToCode]

/test unit-tests