Add overlaybd version number to image-convertor

[WaberZhuang]

What this PR does / why we need it: Add overlaybd version number to both embedded image-convertor and standalone userspace image-convertor. Format as following:

"annotations": {
  "containerd.io/snapshot/overlaybd/version": "0.1.0" // for normal overlaybd-image
  "containerd.io/snapshot/overlaybd/version": "0.1.0-fastoci" // for fastOCI index
}

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 #169

Please check the following list:

• [x] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [ ] 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?