fs: nvs: Allow application to switch sector to get free space

Add an API function allowing the application to determine the amount of
free bytes in the current sector.

Add a second API function allowing the application to switch to the next
NVS sector, calling the garbage collector on such sector.

The goal is togive more granularity to the application to control when
the garbage collector is triggered.

Signed-off-by: Adrien Ricciardi <aricciardi@baylibre.com>