VM Image Controller, Webhook and Download/Upload Refactor for Various Backend Support

[WebberHuang1118]

Code Structure:

• Backend interfaces:
  • pkg/image/backend
• Common VM Image operation package
  • pkg/image/common
• Backing image implementation
  • pkg/image/backingimage
• CDI implementation skeleton
  • pkg/image/cdi

Problem:
VM Image should support various source backend

Solution:
Enhance VM Image implementation to have a clear interface for different image backend

Related Issue:
#6936

Test plan:

• Prerequisites
  • Building Harvester Cluster with the master-head branch ( 0cea310be669040bc682c75edd40fd6f81051bce in my env)
  • Updating VirtualMachineImage CRD
    • $ kubectl apply -f deploy/charts/harvester-crd/templates/harvesterhci.io_virtualmachineimages.yaml
  • Updating harvester deployment with this PR
  • Updating harvester-webhook deployment with this PR
• Case 1: Creating VM image with upload
  • Uploading an OS image file from local
  • Checking the VM image could progress to the completed status
  • Creating a VM with the VM image
• Case 2: Creating VM image with download
  • Downloading an OS image file from a certain URL
  • Checking the VM image could progress to the completed status
  • Creating a VM with the VM image
• Case 3: Creating VM image with export
  • Creating a volume with an existing OS VM Image
  • Exporting the volume as a new image
  • Checking the exported VM image could progress to the completed status
  • Creating a VM with the VM image
• Case 4: VM Image encryption
  • Creating a secret and sc for encryption
  • Creating an encrypted VM image from an existing VM image (non-encrypted)
  • Checking the encrypted VM image could progress to the completed status
  • Creating a VM with the VM image
• Case 5: VM Image decryption
  • Continuing from the above steps
  • Creating a VM image by decrypting the encrypted VM image
  • Checking the decrypted VM image could progress to the completed status
  • Creating a VM with the VM image
• Case 6: VM Image Downloading
  • Triggering downloading an existing VM image
  • The VM image should downloaded as a compressed file

** Additional Context **
Since the VM image implementation has the new interfaces, the original unit testing is not applicable and removed. Creating a separate ticket to track the unit testing implementation #7204