IPsec: add support for DNS search domains in address pools

[tkuehling]

Important notices

Before you submit a pull request, we ask you kindly to acknowledge the following:

• I have read the contributing guidelines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code and text submitted herewith.

If AI was used, please disclose:

• Model used: ChatGPT

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Describe the problem

This change exposes the strongSwan domains pool option in the OPNsense IPsec GUI.

The backend already supports arbitrary pool attributes, therefore no PHP or configuration generator changes are required. This PR only extends the MVC model and the pool dialog.

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Describe the proposed solution

The domains option configures the IKEv2 Configuration Payload INTERNAL_DNS_DOMAIN, allowing clients to automatically receive one or more DNS search domains.

This enables clients to resolve short host names such as nas, printer, fileserver instead of requiring fully qualified names.

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Related issue

No issues were created, because this feature was missing.

[Monviech]

I also use ChatGPT frequently for coding, did it really suggest a TextField here?

Please add proper validation.
https://docs.opnsense.org/development/frontend/models_fieldtypes.html

[Monviech]

This is a default and can be removed