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

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

Merge type

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

[TheRealFalcon]

Thanks @bin456789 ; nice work here!

Only thing left to resolve is this branch needs a rebase against main to fix a merge conflict. The conflict is due to additional tests in test_networkd.py, so it should be a straightforward fix.

[bin456789]

rebased

[TheRealFalcon]

LGTM!