Distinguish cases for Blob provider user update prevention

Since NXP-18059 the Google Drive and Dropbox blob providers don't prevent user updates by default.

This is problematic for Nuxeo Drive as it will triy to synchronize a Google doc Blob that:

• Doesn't have a digestAlgorithm, yet it seems to always be MD5.
• Will fail to be downoaded by Nuxeo Drive since it gets the Google authentication HTML page, this needs to be investigated.
• Should not be overwritten by Drive in case of local modification.

For now we have hardcoded the exclusion of the blobs provided by GoogleDriveBlobProvider and DropboxBlobProvider from the Nuxeo Drive synchronized documents, see NXP-18612.

But we need to find a better way to flag these 4 different cases:
1. JSF UI for atatched file
2. File Importer
3. Drag&Drop
4. Nuxeo Drive
instead of the single preventUserUpdate flag.