bluetooth: host: smp: Add bondable flag overlay per connection

The current API for changing the bondable mode uses the global flag.
With Zephyr support for multiple Bluetooth identities, the API for
changing the bondable mode should be more fine-grained.
The bondable requirements of one identity should not have an impact on
another identity which can have a different set of requirements.
This change introduces function to overlay bondable flag per
connection.

Signed-off-by: Mateusz Kapala <mateusz.kapala@nordicsemi.no>