Bluetooth: shell: gatt: Add discover command

This adds discover command which can be used to discover all attributes
using BT_GATT_DISCOVER_ATTRIBUTE:

uart:~$ gatt discover
Discover pending
Descriptor 2800 found: handle 1
Descriptor 2803 found: handle 2
Descriptor 2a00 found: handle 3
Descriptor 2803 found: handle 4
Descriptor 2a01 found: handle 5
Descriptor 2800 found: handle 6
Descriptor 2803 found: handle 7
Descriptor 2a05 found: handle 8
Descriptor 2902 found: handle 9
Descriptor 2803 found: handle a
Descriptor 2b29 found: handle b
Descriptor 2803 found: handle c
Descriptor 2b2a found: handle d
Descriptor 2803 found: handle e
Descriptor 2b2b found: handle f

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