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
This PR is the final change, where all backends are now purely implemented with async interfaces.
The code changes here replace the LIST call with an async version. This affect all backends (and then some) but the changes are mostly mechanical rewrites of the code.
In many places, the backeds were already prepared for async output, in other some glue was needed.
To reduce the scope of this change, some backends simply report a synchronous result as an async list.
Without await, the using statement can dispose the Stream before the
call to PutAsync completes, resulting in an ObjectDisposedException.
This fixes#4556.
Kenneth Skovhede