Describe the bug
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To Reproduce
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1. Go to 'https://react-query.tanstack.com/examples/nextjs'
2. Click on 'Nextjs SSG'
3. Scroll down to '....'
4. 502 Bad Gateaway

Expected behavior
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Right now they're untrackable

Coordinate with #1133 and #1137

Description of issue or feature request:
For consistent use of OOP we should provide classes for all complex metadata fields
and not only for the outer metadata signature wrapper and the inner signed metadata.

This would make the used metadata format more explicit and might have a positive effect on metadata schema validation (see discussion in #1130)

If CPM_SOURCE_CACHE is set to an empty string, CPM fails with file DOWNLOAD error: cannot create directory '/cpm' - Specify file by full.

The demo apps are not straight-forward, a very basic hello world example would be great :)
thanks

pipupgrade --verbose (0|1|2..) for info, warning, error, etc.