Bluetooth: Shell: Add GATT command get

This introduces get command to GATT which can be used to read the
local attributes:

uart:~$ gatt get 0x000b
attr 0x004235a8 uuid 2a00 perm 0x09
00000000: 74 65 73 74 20 73 68 65 6C 6C

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