altText lost during dataset import

[sourceful-david]

Describe the bug
When running a dataset import the altText and description fields don't get preserved. The altText is definitely there in the tar.gz file we're importing - it can be seen in the assets.json file of the unpacked tar.gz.

To Reproduce
Steps to reproduce the behavior:

1. Run sanity dataset export [source-dataset] on a dataset that has image assets with altText
2. Run sanity dataset import [file] [target-dataset] --replace
3. Find the imported asset in the target dataset and notice that the altText has been lost in the image asset data.

Expected behavior
I know the media plugin is not part of the core sanity sdk but it is heavily promoted and it would be nice for dataset imports to preserve the altText field.

Screenshots

Plugin and environment information:

• Plugin version 2.2.4
• Sanity version @sanity/cli (global) 3.19.1 (latest: 3.19.2)
• Operating system and version macOS 11.5.1