Consolidate `onmetal` and `IPAM` plugins

[damyan]

Currently the onmetal plugin leases an in-band IP address in the IronCore network, the persistency in kubernetes is ensured by the IPAM plugin creating ip.ipam.metal.ironcore.dev objects.

Consolidate the two plugins into a new one, e.g. in-band plugin. The kubernetes IPAM integration shall be made configurable with a flag