Introduce DHCPv6 PXE Support for DPU Metal Case

[guvenc]

Summary

In DPU Metal case, dpservice runs on the DPU and the metal machine running on top. gets one
single interface which is under dpservice's control on the DPU side.
In this setup, the metal machine should be able PXE boot using IPv6.

• Implement (i)PXE DHCPv6 changes in dpservice.
• Implement (i)PXE DHCPv6 automated tests in pytest/scapy.
• Run real life tests and boot a metal machine via IPv6 (i)PXE with DPU and dpservice
• Fix upstream iPXE RA handling.

Connected PRs:

• Introduce DHCPv6 (i)PXE Support #501
• [ipv6] Support/Use DHCPv6 assigned address if there is no Prefix option in RA ipxe/ipxe#1163