interfaces: convert GRE configuration to MVC/API

[fichtner]

Important notices

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

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

GRE interfaces can not be operated by API.

Describe the solution you like

Time to migrate!

Describe alternatives you considered

N/A

Additional context

N/A