Save entities using the schema returned from AI provider

This is ready for review. Here are some QA steps:

This assumes that you already have the module set up in a local sandbox.

1. Check CONTRIBUTING.md and make sure you have enabled all of the modules listed in #10. Especially note serialization has been added.
2. Clear cache. ddev drush cr
3. Attempt to run a sample migration: ddev drush migrate:import slice1_content_with_passthrough
4. Confirm there are no errors with the migration
5. If you want to re-run the migration, roll it back first: ddev drush migrate:rollback slice1_content_with_passthrough
6. After your first run, the AI response is cached. Migrations that are run after that will work with the same AI response. Clear the cache if you would like the migration process to ping the AI provider again.

Looks good @majorrobot approving and merging

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