[CDF-25977] 😅 Extend migration model (#2097)

# 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).
This is step 3 of that process.

This PR introduces `SpaceSource` to track were instances spaces were
created from. This is needed when we migrate
assets/events/timeseries/files/sequences and do not specify each
resource to migrate, but instead use a dataset or hierarchy. In
addition, we need it when we are going to migrate Groups.

<img width="1001" height="628" alt="image"
src="https://github.com/user-attachments/assets/0fad0d6c-7915-40b3-b4c7-2e0609113c32"
/>


## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Changed

- [alpha] The migration model deployed with `cdf migrate prepare` now
includes a view to track dataset migration to instance spaces.

## templates

No changes.
