Do not rely on `SystemIP` to serve IPXE

[defo89]

What would you like to be added:

Currently we serve the appropriate ipxe script based on source IP address the HTTP request was coming from.
Here: https://github.com/ironcore-dev/boot-operator/blob/main/server/bootserver.go#L93-L118

This sets a dependency on assigning same static IP address to the server during initial discovery and during further redeployment.

We could investigate whether UUID or MAC address could be used to identify the server.

[hardikdr]

I like the idea of using UUID as it's already part of the initial request. This can also help us deliver the custom discovery ipxe/ignition while onboarding the machines.
cc @gehoern @stefanhipfel @afritzler