[CDF-25552]⛓️‍💥Support purge with unlink files (#1850)

# Description

**Context**: This is part of the `alpha` migration tooling in Toolkit,
for overview [see
here](https://cognitedata.atlassian.net/wiki/spaces/~170890702/pages/5455773697/Migration+Tooling+in+Toolkit).
T

This can be used to cleanup mishaps on step 3 were the user migrate
files, but want to undo the operation without losing the asset-centric
file and its content.


## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Added

- [alpha] In the `cdf purge instances` you can now unlink files

## templates

No changes.
