Fixes wrong thumbnail field in asset-information

[aaronzi]

This pull request updates the handling and naming of thumbnail information within asset information structures in both the codebase and the integration tests. The main change is the renaming of the thumbnail field to defaultThumbnail for conformance to the spec.

Thumbnail field renaming:

• The field thumbnail in the assetInformation map is renamed to defaultThumbnail in the output of both getAssetAdministrationShellMapByDBID and getAssetAdministrationShellMapsByDBID functions in aas_database.go. [1] [2]

Test updates for new field name:

• All relevant integration tests in aasrepository_integration_test.go are updated to expect assetInformation.defaultThumbnail instead of assetInformation.thumbnail, including test names and assertions. [1] [2] [3] [4]