[CDF-25905, CDF-25906] 😄 Download/Upload instances (#2086)

# Description

**Context** We are working towards adding a new data plugin in Toolkit.
This plugin is to support download, upload and purging of data.

This PR adds support for the downloading and uploading of instances. 

## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Added

- [alpha] New command for downloading instances, `cdf download
instances`. Note that `cdf upload <path>` now also supports uploading
instances.

## templates

No changes.
