[cinder-csi-plugin] Support for arbitrary volume metadata in storageclass parameters

/kind feature

Currently, the cinder-csi-plugin supports a restricted set of StorageClass parameters. There is no mechanism to inject arbitrary Cinder Volume Metadata during the CreateVolume workflow.

In our environment, we use a sidecar agent that watches for volume attachments and enforces iotune values on the libvirt layer based on volume metadata. Using OpenStack Volume Types for this is non-viable because:
- Type Explosion: Managing hundreds of types for different IOPS/Billing permutations is unscalable.
- Performance Impact: In many OpenStack backends, a retype (to change QoS/Metadata) triggers a physical migration of data. Performing a data migration on a volume that is already performance-starved just to update a metadata hint is counter-productive.

Describe the solution you'd like
I propose adding a metadata parameter to the StorageClass. These key-value pairs should be parsed and passed into the Metadata field of the volumes.CreateOpts struct when the driver calls the Cinder API via Gophercloud.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: gold-tenant-a
provisioner: cinder.csi.openstack.org
parameters:
  type: high-performance
  # Proposed metadata injection
  cinder.xxx/static-metadata/x-iops-limit: "5000"
  cinder.xxx/static-metadata/x-billing-code": "34FDVX"

Describe alternatives you've considered

• Unique Volume Types: Too rigid; causes unnecessary physical data movement during QoS adjustments.

The underlying gophercloud/openstack/blockstorage/v3/volumes package already supports Metadata map[string]string in CreateOpts.

I'm willing to contribute to this PR, if everyone agrees it in principle to this feature.

Trace the cinder-csi-plugin CreateVolume workflow and inspect how StorageClass parameters become gophercloud volumes.CreateOpts. Confirm the metadata parameter convention and ensure arbitrary key-value pairs reach CreateOpts.Metadata during volume creation; done means the requested metadata is present on newly created Cinder volumes.