Align Boot Operator with Ironcore OCI Specifications

[hardikdr]

What would you like to be added:

The boot-operator should be adapted to support the latest OCI specification as published by the https://github.com/ironcore-dev/ironcore-image project.
This includes:

• Initially supporting the flat OCI spec
• Later extending support for the architecture-aware, layered OCI spec

Why is this needed:
Brings the boot-operator in line with evolving image specifications within ironcore platform.