Bluetooth: tester: Implement the GAP_SET_MITM command

Implement the BTP command to set MITM, although on Zephyr this is not
configurable at runtime. Instead print a warning instructing to use the
CONFIG_BT_SMP_ENFORCE_MITM Kconfig option instead.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>