Bluetooth: GATT: Add LE Secure Connections Permission Check

Added possibility to enforce that a LE Secure Connection is required
for reading and writing characteristics, i.e. providing a more fine
grained check that allows services that requires secure connection
to co-exists with those that doesn't.

Signed-off-by: Lars Knudsen <larsgk@gmail.com>
Signed-off-by: Fredrik Danebjer <fredrik@danebjer.com>