Bluetooth: Shell: Add GATT command set

This introduces set command to GATT which can be used to write local
attributes:

uart:~$ gatt set 0x000b 62 6c 61 68
attr 0x004235a8 uuid 2a00 perm 0x09
00000000: 62 6C 61 68

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>