Add support for routing functions to the API

This merge request implements the change to the IX-API schema, as decided in the IX-API working group meeting in Frankfurt (2023-10-12).

While implementing and discussing it with @mislam37, @Verzeni, @owais.shaikh and @ahannig we found that there is a substantial overlap between the already merged l3_peer network feature config and the l3_config property.

In order to keep API integrity we need to resolve this inconsistency. This can be done by merging and normalizing the properties into either the network feature config or the l3_config object.