settings: add new API function settings_get_val_len()

Add a function to get the value's length of a Key. If it doesn't exist
returns 0.
Add ZMS implementation for csi_get_val_len() and a default implementation
for the other storage systems.
Add some functional tests to verify it.

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>