Bluetooth: GATT: Allow application to add subscription.

Allow the application to add subscription without resubscribing.
This is needed for persistent bonds that can go to deep sleep and
turn of the RAM or power-cycle.
In this case the application as a GATT client must have a way to add
the subscription callbacks back to the stack before the GATT server
sends notifications.
This should preferable be able to be done before even connecting
since the notifications can arrive immediately after connecting to
the peer.
The stack cannot persist this on it's own since it must remember the
function pointers for the callback functions. Storing these in
persistent storage would not be compatible with a Device Firmware
Upgrade (DFU) solution.

Fixes: #21798

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