Allow EC2 Elastic IP association with a network interface

[ishu7w]

Description

Fixes #1799. EC2 instances with multiple network interfaces cannot use the current Elastic IP association method: it always sends InstanceId, while AWS requires NetworkInterfaceId for that case.

Add an optional network_interface parameter accepting the existing EC2NetworkInterface type. When supplied, send its ID instead of InstanceId; existing calls retain their request parameters. Forward the option through the deprecated alias and document its use with VPC addresses.

AWS contract: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_AssociateAddress.html

Validation

• The new regression fails on unchanged trunk because the interface option is unsupported.
• All 1,211 EC2 tests pass. Request assertions verify interface targeting, absence of InstanceId, allocation ID use, legacy instance targeting, and alias forwarding.
• Compute test suite: 5,920 passed, 5 skipped, 3 subtests passed; libvirt test module excluded because its native dependency is unavailable.
• Black, isort, whitespace, and ASF license checks pass. Flake8 reports only the unchanged, pre-existing unused typing.List import in ec2.py (also verified against trunk).
• AWS requests are fixture-tested; no live cloud resources were created.

Status

Ready for review.

Checklist

• Code formatting and import sorting
• Documentation
• Tests
• ICLA (if maintainers consider this extension a bigger change)

AI assistance: This implementation, regression tests, and description were prepared with OpenAI Codex.