--namespace should accept a list of values

[shapirus]

What would you like to be added:

Currently, as specified in the docs, the --namespace command line argument can either be empty, to mean all namespaces, or contain a single value to mean exactly one namespace.

It should be extended to support taking a list of values, either comma-separated or space-separated, or by specifying the option multiple times. Neither will break existing configurations, so the code modification should be trivial.

Why is this needed:

To avoid having to create multiple deployments of external-dns when it has to operate on more than one namespace, but less than all.

[ivankatliarchuk]

I'll assign it to myself.

This is quite a lot of work so not sure when going to have time

/assign

[ivankatliarchuk]

Found this PR https://github.com/kubernetes-sigs/external-dns/pull/3533, looks like someone already tried

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/lifecycle stale

[shapirus]

/remove-lifecycle stale

[diwakar-dubey]

the previous PR wasn't merged, @ivankatliarchuk , do you think we can have that. It'd be really cool to have this.