stream_flash: Enforce size to be explicitly present on init

The commit changes requirements for stream_flash_init, where size
can no longer be 0 and has to be explicitly set, to avoid situation
where size autodetection, invoked by size == 0, would miss changes in
layout and silently allow overflow of Stream Flash into other partitions.

There has also been new Kconfig option CONFIG_STREAM_FLASH_INSPECT,
set to y by default to keep legacy behaviour, that can be used to turn
off stream_flash_ctx vs device inspection, allowing user to shed
inspection code once it is not useful anymore.

Fixes: #71042

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>