Bluetooth: Host: Add CONFIG_BT_BONDING_REQUIRED flag

Added configuration for accepting pairing requests only if both devices
has bonding flag set in order to reject other devices at an early stage,
thus leaving more chance for devices expected to bond.

With the CONFIG_BT_BONDING_REQUIRED flag the device only accept pairing
requests if it has CONFIG_BT_BONMDABLE set and the device requesting
pairing has Bonding_Flags field set to Bonding (0x01) in its AuthReq.
Note: When using bt_set_bondable(false) pairing requests will be
rejected when CONFIG_BT_BONDING_REQUIRED is set.

Signed-off-by: Martin Rieva <mrrv@demant.com>