Add ClusterAlert and ClusterProvider resources

[rossigee]

Describe the bug

To get alerts from all namespaces, we have to configure the following extra resources into each namespace:

• Alert
• Provider
• Provider's Secret

In many organisations this information is the same for each namespace, so there is a lot of duplication and extra work required to configure it.

Perhaps a ClusterAlert and/or ClusterProvider resource would be useful?

Steps to reproduce

N/A

Expected behavior

To be able to define a single ClusterProvider and ClusterAlert which would be available to all namespaces.

Would be nice: An attribute where you could specify a namespace inclusion/exclusion filter.

Screenshots and recordings

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OS / Distro

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Flux version

N/A

Flux check

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Git provider

N/A

Container Registry provider

N/A

Additional context

N/A

Code of Conduct

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