fix(net/networkd): add GatewayOnLink flag when necessary

[bin456789]

Proposed Commit Message

fix(net/networkd): add GatewayOnLink flag when necessary

When the gateway isn't part of the subnet's network, the "GatewayOnLink" flag is required for the route to work.

Fixes GH-4995

Additional Context

Test Steps

Checklist

• [x] My code follows the process laid out in the documentation
• [x] I have updated or added any unit tests accordingly
• [ ] I have updated or added any documentation accordingly

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)