Support dynamic NAT pools for router in Elektra

[sebageek]

Our asr1k driver now supports adding multiple IPs to the external gateway of a router. This will result in a NAT pool being created in the router config, resulting in a larger NAT pool, which is a feature a couple of customers have requested. We would like this feature to be exposed in Elektra.

From an API perspective this works by providing multiple external_fixed_ips as part of the external_gateway_info. This can be done on router create as well as on router update. From an API perspective "having a Pool" looks no different than "three or more IPs on the external gateway".

Caveats:

• two ips are not possible. either provide one ip or three (two for the pool, one for the router)
• the last IP is always used for the router itself
• all pool ips need to be consecutive (but if only a subnet id is provided the driver autoallocates the pool)

The driver will complain about anything that is not working with a HTTP Bad Request. The description + implementation can be found here in the driver including all new exceptions the driver can throw. Details on the feature can be found in our internal ADR repo or soon in our user docs.