Check the expected `OCI` layers before booting a `Machine`

[lukasfrank]

What would you like to be added:

Currently the boot of a Machine fails if an OCI image is passed with invalid/not expected layers.
For the user it would be helpful if the image format would be validated upfront (without booting a Machine) and an error should be attached to the reflecting resource.
Additionally, we can emit an event and attach it to the ServerClaim.

[afritzler]

We should not set a ServerBootConfiguration to a Ready state when the OCI image is incorrect. Also I would emit an event on the ServerBootConfiguration as the boot-operator should not know anything about a ServerClaim.