This PR adds a dynamic property so a backend can signal if it supports streaming, based on the settings.
This is currently used for the File backend, so that toggling `--use-move-for-put` will disable streaming on the backend instead of relying on the `--disable-streaming-transfers` flag.
This PR moves the check from the FTP backend into shared code, so all backends will now check for read/write permissions when using the "Test" button.
For situations where write permissions are not required (like restore or verify) there is a flag on the v2 API that indicates if the connection was established, so the UI can show a message suggesting that it is possible to proceed, if the current UI operation does not require write permissions.
This fixes#2473
Kenneth Skovhede