net/firewall: Gateway Groups not available

[ansibleguy]

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Describe the bug
Plugin version used: 1.2
The os-firewall automation-rules do not allow gateway-groups to be selected (as gateway).

If one tries to set a gw-group via API there is no error response.
The gateway of the processed rule is set to 'none'.

To Reproduce
Steps to reproduce the behavior:

1. Add a gateway group in 'System - Gateways - Group'
2. Install the plugin os-firewall
3. Try selecting the before created gw-group as target gateway for a rule in 'Firewall - Automation - Filter'

Expected behavior
Being able to select gateway groups as target gateway for those rules as this is an important functionality.

Screenshots
Per example:

1. automation rules
   
2. default rules
   

Additional context
This is a very commonly used functionality and therefore it would be nice for it to be available using the API.
Lack of this functionality makes the API management unusable in many practical use-cases and environments. (WAN-/VPN-Failover and so on..)

[sbobber]

Same issue on:
OPNsense 22.7.7_1-amd64
FreeBSD 13.1-RELEASE-p3
OpenSSL 1.1.1s 1 Nov 2022

[AdSchellevis]

@sbobber can't be an issue if it's a feature request, but feel free to open a PR

[sbobber]

@AdSchellevis understood, would be great, thanks

[OPNsense-bot]

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