Oracle datasource: secondary VNICs should use DHCP + routing-policy like EC2

[qm3ster]

On OCI, secondary VNICs currently get static-only config. This means:

• Only ipv6Addresses[0] is configured, additional IPv6 addresses are dropped (DHCPv6 gets enabled only on no-IPv4 primaries, which does yield all configured addreses)
• No routing-policy rules, so outbound traffic from secondary VNIC IPs exits the primary interface and gets dropped by OCI's anti-spoofing (this affects IPv4 and IPv6)
• Users must hand-write a second netplan file for routes, routing-policy, and additional IPs

Tested on VM.Standard.A1.Flex (Ubuntu 22.04, 24.04):

• OCI does not serve DHCPv4 on secondary VNICs (tested on multiple instances, no response)
• OCI serves DHCPv6 on all VNICs — secondary VNICs get all assigned IPv6 addresses automatically
• OCI's IMDS reports all IPv4 and IPv6 addresses per VNIC, plus subnet CIDRs
• Routing-policy (from: <ip>, table: <N>) is required for secondary VNICs or outbound traffic is dropped

The EC2 datasource already handles all of this (DataSourceEc2.py:1065-1186):

• dhcp4: true on every NIC, not just primary
• dhcp6: true when IPv6 addresses exist
• Per-IP routing-policy rules on non-primary NICs with table: 100 + nic_idx
• Additional static addresses via get_secondary_addresses()
• Single-NIC cleanup (strips unnecessary overrides)

Oracle's datasource could follow the same pattern, adapted for OCI's DHCP behavior (no DHCPv4 on secondary, DHCPv6 on all).

We have a working v2-based implementation locally. Happy to PR if there's interest.

[qm3ster]

I made #6843 for the actual bug in the meantime, a tiny change.
But having all of the IPs apply automatically would be better.
I propose going the DHCP direction for as much as possible (first IPv4 on primary NIC and all IPv6 everywhere) since OCI already distributes those, and the current provider uses dhcp on for the first IP of the primary interface.
But since DHCPv4 at a standards level (and in OCI's implementation) doesn't distribute more than one IPv4 address, the script should emit static Address = entries for all queried IPv4 addresses except the first on the primary NIC.