Allow passing volume tags through PVC annotations

[rdpsin]

There's a similar related issue here: #86

This issue proposes passing a SINGLE PVC annotation to the CSI Driver as part of CreateVolumeRequest.Parameters to allow CSI drivers to tag volumes with custom tags provided at the PVC level.

When creating a PVC, a user can specify tags to be attached to their volumes (if the driver supports it) by creating an annotation: csi/additional-volume-tags, which is a comma separated list of key-value pairs.

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: pvc-1
  annotations:
    csi/additional-volume-tags: "key1=value1,key2=value2"
spec:
    ...

The feature can be gated by an OPTIONAL annotation in the StorageClass, which defaults to false.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: sc1
  annotations: 
     csi/allow-additional-volume-tags: true
spec:
     ...

Volume tags will be appended to CreateVolumeRequest.Parameters if and only if the annotation is set to true. CSI drivers are free to ignore this annotation if they don't support tagging.

I'm willing to work on a PR if the community thinks this change makes sense.

[xing-yang]

Similar request was brought up before but didn't get accepted because it breaks PVC portability:
#425 (comment)

[xing-yang]

cc @msau42