Add support for multiple gloo namespaces in one External DNS instance

[megum1n]

Description
Adding support for multiple gloo namespaces in one External DNS instance.

Example:

external-dns --source gloo-proxy --gloo-namespace gloo-system-1,gloo-system-2
external-dns --source gloo-proxy --gloo-namespace gloo-system-test

Fixes #3293

Checklist

• Unit tests updated
• End user documentation updated

[esalter]

I'm not sure changing the name is the best idea. Isn't it easier to just change the behavior and document that it takes a comma-separated list?

[megum1n]

@esalter I'm fine with ether way as long as it does the thing. I just thought that new name would be easier to understand without even reading the docs.
Let's wait for maintainers to review this PR and decide what is the best way to name this flag. It will be easy fix to rename flag back to gloo-namespace if we all agree that this is the best option.

[szuecs]

I think as long you don't break the users that have only one namespace you can update the behavior and don't change the names of the variables. Please make sure by a test that one namespace works as before and the new feature also works.

[megum1n]

@szuecs I used strings.Split, so it will work the same way for one namespace or for multiple namespaces.
https://pkg.go.dev/strings#Split

If s does not contain sep and sep is not empty, Split returns a slice of length 1 whose only element is s.

[szuecs]

Then make sure the user interface does not change because otherwise you will break users if they upgrade.

[megum1n]

@szuecs I changed flag name back to gloo-namespace. This PR now won't bring breaking changes.

[szuecs]

/ok-to-test

[szuecs]

level=warning msg="[linters_context] rowserrcheck is disabled because of generics. You can track the evolution of the generics support by following the golangci/golangci-lint#2649."
source/gloo_proxy.go:88: unnecessary leading newline (whitespace)
func NewGlooSource(dynamicKubeClient dynamic.Interface, kubeClient kubernetes.Interface, glooNamespaces string) (Source, error) {

make: *** [Makefile:51: go-lint] Error 1
Error: Process completed with exit code 2.

[megum1n]

@szuecs I removed whitespaces. Please run tests again (I checked locally - no errors).

[szuecs]

/ok-to-test

[johngmyers]

/lgtm
/assign @Raffo

[johngmyers]

/lgtm

[megum1n]

@Raffo @szuecs bump

[johngmyers]

/approve

[k8s-ci-robot]

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[johngmyers]

/test pull-external-dns-lint
/test pull-external-dns-unit-test