[Enhancement] [Regions] Add 'AddOrReplace' to IRegionBehaviorFactory

[Luca Civale]

Summary

We want to replace an existing behavior with our custom behavior(this is already possible for region adapters). Currently we have to register custom region adapters to exclude the unwanted behavior(s). It would be a lot easier to customize behaviors if we could replace exising ones. The (old?) docs state that this was possible with the ConfigureDefaultRegionBehaviors but I didn't find this method.

API Changes

In order to replace behaviors the IRegionBehaviorFactory should expose the new AddOrReplaceIfMissing method.

.ConfigureRegionBehaviors(behaviors =>
{
    behaviors.AddOrReplaceIfMissing<CustomBehavior>(BehaviorToReplace.BehaviorKey);
})

A default implementation should be added also to IRegionBehaviorFactory to prevent breaking changes.

Intended Use Case

We need to replace the RegionManagerRegistrationBehavior to enable multiple instances of the same region with view discovery.

[Luca Civale]

@dansiegel I can create a PR for this if it's approved

[Dan Siegel]

I want to make sure that the API's stay in sync as much as possible between the various platforms. If you want to do a PR we would certainly welcome it... but we need to make sure that we have test coverage for this across both WPF and MAUI.

[Luca Civale]

Started the implementation and saw that default interface implementation isn't supported accorss all targeted runtimes. Therefore this would be a breaking change. Can I still implement it as proposed or do we need another approach?