This adds bt_gatt_read which can used to read attribute values.
Note: Attributes which values are bigger than the MTU need to be read
multiple time until the end since the stack cannot cache any data.
Change-Id: I09c502eeb674a577673182c35a0e1a84519fd3b7
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>