kexec from the agent

[smira]

This contains two changes:

• Sidero always PXE boots the agent (if the server reaches out for PXE boot)
• If Talos is already installed, Sidero agent kexecs into the installed Talos (by reading GRUB configuration)
• if server has to wiped, registered, etc., agent performs requested action and goes into long-polling mode
• once server is allocated, agent downloads kernel + initramfs from Sidero and kexecs into it to start the installation process

(As Talos does kexec as well, it means that server can be provisioned without a single reboot from agent straight into Talos)

• [ ] #610
• [ ] #611
• [ ] #612
• [ ] #613
• [ ] #614