Provide option to preserve filename case while downloading assets.

Problem/Motivation

After running drush acquia-dam:update-assets --media-type=<type> to migrate from the old Media: Acquia DAM module to the new Acquia DAM module, all document filenames are converted to lowercase. The customer needs original mixed-case filenames displayed on their frontend.

What we found:

• The module's getValidFilename() method applies strtolower() to filenames during the migration/update process.
• The field mapping UI only exposes these source fields by default: Created date, Deleted date, Expiration date, File upload date, File name, Last updated date, Release date, Size.
• "File name" in the mapping references the local Drupal file entity, which has already been lowercased by the module.
• The customer discovered that enabling "Title" via config/acquia-dam/metadata makes it available in the mapping UI. However, Title and the original filename are often different values in their DAM, so this is not a reliable substitute.

Proposed resolution

Provide way to preserve filename case.