Firewall: merge outbound NAT into SNAT page

[Monviech]

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Describe the problem

Fixes: #9515

For the Source NAT a few things would be needed:

• The controller could emit the automatic rules inside the searchRuleAction() so they are visible in the frontend
  • Hide either automatic, or manual rules from what searchRuleAction returns, depending on snat_mode
  • Add a banner when NAT mode is disabled and hide grid completely
• Some endpoint to change the Source NAT modes
• No automatic migration, offer export/import of rules
  • Import/Export of rules in MVC NAT pages: Firewall: NAT: download/upload rules as csv #10371
  • Export of rules in Legacy Outbound page:
• Hide Outbound Rules when no rules are defined inside it and its set to automatic rule generation / move to legacy plugin