feature: Add gateway parameter in peers

[ElektronikerDev]

As described in https://github.com/natesales/pathvector/issues/197 , it should be possible to set the gateway mode in peers, in which the gateway is determined.

The strings 'direct' or 'recursive' can be assigned to the gateway parameter.

Details to the "gateway" functionality are also described in the bird documentation: https://bird.network.cz/?get_doc&v=16&f=bird-6.html