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

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: , table: `) 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.