backend/sftp: Add sftp.args option

Allow setting custom arguments for the `sftp` backend, by using the
`sftp.args` option. This is similar to the approach already implemented
in the `rclone` backend, to support new arguments without requiring
future code changes for each different SSH argument.

Closes #4241

doc/030_preparing_a_new_repo.rst

@@ -119,10 +119,10 @@ user's home directory.
 Also, if the SFTP server is enforcing domain-confined users, you can
 specify the user this way: ``user@domain@host``.
 
-.. note:: Please be aware that sftp servers do not expand the tilde character
+.. note:: Please be aware that SFTP servers do not expand the tilde character
           (``~``) normally used as an alias for a user's home directory. If you
           want to specify a path relative to the user's home directory, pass a
-          relative path to the sftp backend.
+          relative path to the SFTP backend.
 
 If you need to specify a port number or IPv6 address, you'll need to use
 URL syntax. E.g., the repository ``/srv/restic-repo`` on ``[::1]`` (localhost)
@@ -174,7 +174,11 @@ Last, if you'd like to use an entirely different program to create the
 SFTP connection, you can specify the command to be run with the option
 ``-o sftp.command="foobar"``.
 
-.. note:: Please be aware that sftp servers close connections when no data is
+The SFTP backend has the following additional option:
+
+ * ``-o sftp.args`` allows setting the arguments passed to the default SSH command (ignored when ``sftp.command`` is set)
+
+.. note:: Please be aware that SFTP servers close connections when no data is
           received by the client. This can happen when restic is processing huge
           amounts of unchanged data. To avoid this issue add the following lines 
           to the client's .ssh/config file: