Verify checksums of images imported via external_source_image_url

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#### 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.

索引モデルが issue の本文から書いたものです。