babel: add API route to ip assignment that returns all IPs based on one IP of the node

[Noki]

An automatic migration path from OLSR to babel requires a way to automatically determine IPv6 IPs assigned to nodes so they can be added to nodes that are currently IPv4 only. This issue depends on #76 and #75 .

[nicolasberens]

Random thoughts:

we might need the following endpoints:

request_ip:
receives the data that user enters into the wizard, returns a token, sends email to user to validate

check_nodename:
receives the name of the node, returns if its available (so that we can check if the name is available while typing)

get_ip:
router can query this endpoint with its token, when email was validated the endpoint returns the configuration

[Akira25]

As already mentioned on the meeting at c-base, I started by writing a self-documentory API:

https://github.com/freifunk-berlin/config.berlin.freifunk.net-API/tree/main

The current specification file is attached below. We might tweak it more to fit the requirements named by you, though.
openapi.json