Add support to RAG application to create a cluster for the user, or allow them to bring their own cluster

[umeshkumhar]

• Create a cluster with appropriate nodepools/other config on behalf of the user
• Still allow bringing your own cluster
• Update marketplace blueprints with cluster creation options

[artemvmin]

FYI something in this commit changed tf apply (pre-deployment) plan generation from O(seconds) to O(minutes). Unless I made a mistake, we need to solve this or revert this change.

# at HEAD (this commit):
time (tf apply -var-file workloads.tfvars)` 
...
real    5m16.797s
user    8m49.858s
sys     0m22.037s

# at `HEAD~1`
time (tf apply -var-file workloads.tfvars)` 
...
real    0m20.667s
user    0m26.451s
sys     0m3.219s