Support Additional Announcable CIDRs

[majst01]

TODO:

• adopt network tests

When deploying this version of metal-stack, for existing tenant super networks it should be considered to add `10.240.0.0/12` to the new field `additionalAnnouncableCIDRs` to the deployment configuration. This network was hard-coded by default and is now optional. The metal-api will prevent accidental removal of this network, so it is not possible to break existing route maps if this version gets deployed without adding the field to the deployment.