Support networkd RequiredForOnline option

[dankm]

Implement the "optional" ethernet interface property from NetPlan for the networkd renderer. Marks an interface as not required for the network-online target using the RequiredForOnline=no Link property.

Proposed Commit Message

feat(networkd): Support RequiredForOnline link option

This allows an administrator to specify that an interface may not be available at
boot time, so we can avoid hanging while we wait for a configuration that might
never happen.

Test Steps

Using the networkd renderer, add an optional: true property to an interface. See that networkd no longer blocks the boot waiting for it to be configured.

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 for this PR! Overall, everything looks good.

Can you also add the new key to the network config v2 schema? Otherwise, specifying the key will result in a warning.

[dankm]

Not a problem. I thought about adding it the first time, but wasn't sure how to test it, or if it was necessary. Thanks.

[TheRealFalcon]

LGTM! Thanks!