Add Discovery State and rework Initial power/pxe handling

[defo89]

Proposed Changes

• add Discovery state where we expect server to PXE boot and post to registry
• Set PXE boot only once for Initial server and proceed to Discovery state. With this we do not send PXE boot all the time while metal server is booting up
• On the Initial state we power off the server regardless, hence no need to cover server that is powered on For now, we need to power off the server manually if it's on

Fixes #97

[hardikdr]

I suggest we have a quick offline discussion next week before we go ahead here. I have a few points I’d like to cover:

• Could we leverage the NodeConditions like API instead of introducing a new state? This might streamline the integration without altering the state machine significantly.

• The rationale for always using PXEBootOnce is to ensure that the Metal Machine consistently fetches the latest PXE script and Ignition during Initial state. Since the OS runs in-memory, especially with Gardenlinux, avoiding PXE could result in the machine booting from disk, which might lead to unexpected behaviors.

• Regarding the issue with machines that are already powered on, perhaps a hint in the Endpoint object about whether Servers should undergo a Firstboot or be directly moved to Available during onboarding might offer a more holistic solution.

[afritzler]

@defo89 can you also run make docs to regenerate the API reference documentation?

[defo89]

@defo89 can you also run make docs to regenerate the API reference documentation?

thanks for the hint - done