Migrate ownership of IP blocks to NICo-core

[Matthias247]

IP blocks are the highest level of IP resource that NICo manages.
Parts of IP blocks can be leased out to NICo users, which then use the contained prefix ranges to create VPC prefixes and subnets.

Scope of this issue:

• Support IP block definition via either config files or gRPC APIs in NICo-core
• Offer IP block allocation IPs in NICo-core.
• manage the lifecycle of IP blocks if needed. Don't vend out parts of the IP block which might still be in use by other tenants

[DrewBloechl]

I've got an old branch somewhere that IIRC has most of this implementation. I'll see if it can be dusted off and adapted to the current codebase for 2.1.