[FEATURE] Create backing image from volume

[yasker]

Is your feature request related to a problem? Please describe.
For VM users, it's often that the user wants to update the content of the volume then create more VM with the updated content. As the storage layer, Longhorn needs to implement creating backing images from volumes in order to support this use case.

Describe the solution you'd like
Support creating a backing image from a volume.

Describe alternatives you've considered
Download the volume content as a raw image file, then upload it somewhere, and create a Longhorn backing image using the remote URL.

Additional context
Add any other context or screenshots about the feature request here.

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

• Is there a workaround for the issue? If so, where is it documented?
  The workaround is at:

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at: chart: Add backing image options for default StorageClass #2930
  The PR for the chart change is at: chart: Add backing image options for default StorageClass #2930

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at
  Implementing VolumeExport() gRPC in replica SyncAgentServer longhorn-engine#640
  Support exporting backing image from volume backing-image-manager#24
  Support exporting backing image from volume longhorn-manager#991
  Update backing image manager to v2_20210820 longhorn-manager#994

• Which areas/issues this PR might have potential impacts on?
  Area
  Issues

• If labeled: require/LEP Has the Longhorn Enhancement Proposal PR submitted?
  The LEP PR is at

• If labeled: area/ui Has the UI issue filed or ready to be merged (including backport-needed/*)?
  The UI issue/PR is at feat(backingImage) Backingimage support export from volume longhorn-ui#429

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at docs: Update the backing image doc website#359

• If labeled: require/automation-e2e Has the end-to-end test plan been merged? Have QAs agreed on the automation test case? If only test case skeleton w/o implementation, have you created an implementation issue (including backport-needed/*)
  The automation skeleton PR is at integration: Add test skeleton 'test_exporting_backing_image_from_volume' longhorn-tests#679
  The automation test case PR is at
  The issue of automation test case implementation is at (please create by the template)

• If labeled: require/automation-engine Has the engine integration test been merged (including backport-needed/*)?
  The engine automation PR is at

• If labeled: require/manual-test-plan Has the manual test plan been documented?
  The updated manual test plan is at:

• If the fix introduces the code for backward compatibility Has a separate issue been filed with the label release/obsolete-compatibility?
  The compatibility issue is filed at

[innobead]

Waiting for UI integration. cc @smallteeths

[kaxing]

Validation: PASSED

Tested with Longhorn-v1.2.0-rc2

Screenshots for reference:

Popup for creating backing image	Status of backing image created from volume