[develop] add create_snapshot() interface

[BigVan]

What this PR does / why we need it:

Add online snapshot functionality to overlaybd.

// THIS IS UNFINISHED, just to trigger the CI, do not merge it !

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:

• [ ] 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?
• [ ] Do all new files have an appropriate license header?