Feature request: Automatic PVC storage size increase

It would be really cool if the operator could automatically increase the size of PVCs for VMSingle and VMCluster.

This was mentioned way back in an issue here: https://github.com/VictoriaMetrics/operator/issues/161

Lets say we want to retain metrics "forever" (10 years?). If we can automatically "step up" the storage size instead of pre-allocating the expected storage size that would be about a 50% cost reduction.

Another benefit is the peace of mind that if we either miscalculate the storage requirements up front, the data volume increases or we forget to increase regularly it will just manage itself.

I imagine a spec that looks something like:

initialSize: 50G
storageResizeStep: 10G
maxStorageSize: 1000G

Threshold for doing the resize can be once there is less than storageResizeStep space available, to keep things simple.

maxStorageSize exists as a safety mechanism to avoid extreme surprises.

Additionally there should be metrics for storage_size_max and storage_size_current that allows for creating an alert to manually intervene if we're about to reach the maximum size and the automation would no longer help us.

Start by reading the related issue #161 and tracing how the operator reconciles PVCs for VMSingle and VMCluster. Define the configuration and safety behavior for incremental expansion, maximum size handling, and the requested storage metrics, then verify the design against Kubernetes PVC expansion constraints and relevant reconciliation tests.