Add a Target.execute_raw() method that provides raw stderr and stdout output as
bytestring.

This allows dealing with binary data in the output of commands and also
splitting stdout stderr.

Allow not decoding the values and keep them as bytestring, for when e.g.
sysfs files have binary content (e.g. exposed from device tree)

True decodes the bytestring, None attempts to decode and False does not
decode.