net: openthread: cli: Align config of OT and Zephyr shell max cmd size

This PR enables the user to configure the maximum command length for
OpenThread's CLI using Zephyr's shell, which is currently fixed to
256 characteres. This limitation was not enough for certain allowed
OpenThread commands, like some variants of 'ot dataset mgmtsetcommand'.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>