feat(service): add multi-namespace support for Service source

[AryanSharma9917]

What this PR does

Adds multi-namespace support for the Service source by introducing repeatable --namespaces and label-based --namespace-label-selector filtering.

Why

This enables scoped multi-tenant deployments without requiring a separate external-dns instance per namespace.

Highlights

• Adds --namespaces and --namespace-label-selector configuration
• Wires new config fields through source config/store
• Updates service source informer handling for multi-namespace behavior
• Adds validation and test updates
• Updates docs for new flags and usage

Fixes #3565

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[mloiseleur]

Instead of going down this path of having multiple namespaces array options per source, maybe we can consider extending current namespace option to allow multi namespace. It would simplify this PR, cover more use cases and won't require any additional flags.

We may even in a follow up PR deprecate and after remove gloo & gateway specific namespace options.

@AryanSharma9917 Wdyt ?

cc @Raffo @szuecs @vflaux

[AryanSharma9917]

Instead of going down this path of having multiple namespaces array options per source, maybe we can consider extending current namespace option to allow multi namespace would simplify this PR, cover more use cases and won't require any additional flags.

We may even in a follow up PR deprecate and after remove gloo & gateway specific namespace options.

@AryanSharma9917 Wdyt ?

cc @Raffo @szuecs @vflaux

Hi @mloiseleur,

I think that would be a good approach, though I have a few questions before I move forward. Do you prefer a specific format like a comma separated list or an array? Also, should I start deprecating the Gloo and Gateway-specific options in this PR, or wait for the follow-up?

[mloiseleur]

Do you prefer a specific format like a comma separated list or an array?

Comma-separated list.

should I start deprecating the Gloo and Gateway-specific options in this PR, or wait for the follow-up?

Deprecation in a dedicated PR, if that's ok for you ?

[AryanSharma9917]

Do you prefer a specific format like a comma separated list or an array?
Comma-separated list.

should I start deprecating the Gloo and Gateway-specific options in this PR, or wait for the follow-up?

Deprecation in a dedicated PR, if that's ok for you ?

@mloiseleur Thanks for confirming. That works for me.

I will keep this PR focused on multi namespace support using a comma separated list and open a separate PR for deprecating the Gloo and Gateway specific namespace options.

Please let me know if my understanding is correct or if I misunderstood anything.