Bluetooth: Host: Add option to force pairing in bt_conn_security

Add option to force the host to initiate pairing procedure even if the
host has encryption keys for the peer.
This option can be used to pair with a bonded peer that has deleted its
bonding information without deleting the keys. If new pairing results
in weaker keys the pairing will be aborted.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>