fix(content-import-job) fix key fields

[valentinogiardino]

This pull request focuses on updating the content import functionality to use field IDs instead of field names.

Key changes include:

Validation and Schema Updates:

• dotCMS/src/main/java/com/dotcms/jobs/business/processor/impl/ImportContentletsProcessor.java: Updated the field validation logic to use field.id() instead of field.variable().
• dotCMS/src/main/java/com/dotcms/rest/api/v1/content/dotimport/ContentImportParamsSchema.java: Modified the example JSON to use a field ID instead of a field name.

Test Case Adjustments:

• dotcms-integration/src/test/java/com/dotcms/rest/api/v1/content/dotimport/ContentImportResourceIntegrationTest.java:
  • Added fieldId to store the ID of the first field in the content type. [1] [2]
  • Updated test cases to use fieldId instead of the field name "title". [1] [2] [3] [4]

Postman Collection Update:

• dotcms-postman/src/main/resources/postman/ContentImportResource.postman_collection.json:
  • Added a script to set the field ID in the collection variables after creating the content type.
  • Updated the fields variable to be empty initially, to be populated by the script.

This PR fixes: #30669