Part of [[ https://linear.app/comm/issue/ENG-3966/rename-the-holder-message-field-to-bloburi | ENG-3966 ]].
Introduced simple utility functions that should be used always when we want to get blob URI from encrypted multimedia, instead of accessing `blobURI` / `holder` directly.
These functions will be used in the next diff to make this one small and clean
Depends on D8371