[Feature] Support custom images for nodes in AKS managed clusters

[nclaeys]

Is your feature request related to a problem? Please describe. It would be very useful for us if we could customize the images used by the AKS nodes, as already exists on EKS or GKE. We have 2 main usecases for this:

• packaging pre pulled images for our daemonsets/common used images and thus speeding up the startup of a node significantly
• building a custom image with sysbox installed, again to drastically speeding up the startup of these nodes (on eks: 8min -> 3min). Now we need to depend on the sysbox daemonset to configure the node but this is really slow when dynamically scaling nodes.

Describe the solution you'd like Allow nodepools to specify a custom image instead of only supporting the aks images. Open source the building of the aks images such that people can extend them.

Describe alternatives you've considered We looked at the CAPI project but do not want to create non-managed clusters. Other alternatives are hard to come by as the AKS nodes are quite closed source and it is very difficult to figure out how to build an Azure image as well as there is no way to configure custom images in nodepools.