Verify checksums of images imported via external_source_image_url

#### Community Note

Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request.
Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request.
If you are interested in working on this issue or have submitted a pull request, please leave a comment.

#### Description

Add verification of checksums for images imported via external URLs.

#### Potential configuration

```
external_source_image_url = "https://cloud.debian.org/images/cloud/bookworm/20231013-1532/debian-12-generic-amd64-20231013-1532.qcow2"
external_source_image_checksum = "b2ddc01e8d13dabbcfde6661541aae92219be2d442653950f0e44613ddebaeb80dc7a83e0202c5509c5e72f4bd1f4edee4c83f35191f2562b3f31e20e9e87ec2"
```

Start by locating the code that handles the external_source_image_url configuration and review how related image options are parsed. Trace the imported image flow and determine where external_source_image_checksum should be checked; done means a configured checksum is verified and mismatches are handled safely.

Vom Indexierungsmodell aus dem Issue-Text verfasst.