[Feature] Investigate Managed DALEC for fleet image builds

[ytimocin]

Is your feature request related to a problem? Please describe.

Investigate adopting Managed DALEC for fleet image builds.

Describe the solution you'd like

Sync with the team already working on this; file onboard.yml PR into azure-management-and-platforms/aks-dalec-build-defs if applicable; switch build-publish-mcr.yml over once onboarded.

Additional context

Pairs with #1303. Mirror in Azure/fleet-networking.

[britaniar]

Onboarding yamls: https://github.com/azure-management-and-platforms/aks-dalec-build-defs/pull/201/changes/f25039d0ca3c7be9b7abf6bdba73c1f9e027f412