convertor: support reference API

[WaberZhuang]

What this PR does / why we need it: Add option --referrer so that the converted image will be referenced to the original image through Referrers API. The artifact type for overlaybd and turboOCIv1 is application/vnd.containerd.overlaybd.native.v1+json and application/vnd.containerd.overlaybd.turbo.v1+json respectively.

Please see the changes in USERSPACE_CONVERTOR document for more details.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Please check the following list:

• [x] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [x] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [x] Do all new files have an appropriate license header?

[liulanzheng]

change alibaba.overlaybd to containerd.overlaybd?

[WaberZhuang]

change alibaba.overlaybd to containerd.overlaybd?

Already updated as suggested, and since this repository is a containerd subproject, it does make more sense to use containerd.overlaybd.