Enable choosing the interface identifier on interface creation

[bimbar]

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Is your feature request related to a problem? Please describe.

There are two scenarios:

• yesterday a customer lost his interfaces in some way I can't quite fathom and then recreated them in the wrong order causing all firewall rules to apply to the wrong interfaces. The only remedy was to grep the correct assignments out of a restored backup and then delete and recreate all interfaces in the right order.
• Also, in HA scenarios it would be easier to create interfaces, especially on differing hardwares.

Describe the solution you like

On creating an interface, make it possible to either choose the interface id outright, or choose the number X in optX.

Describe alternatives you considered

Creating interfaces in the right order is possible, but especially in the case of non-contiguous numbering, which can happen if an interface is deleted later, this is not very convenient.

Additional context

Discussed it with franco: https://forum.opnsense.org/index.php?topic=40985.0;topicseen