mgmt/mcumgr/lib: Fix parasitic use of heap by image management

The commit fixes issue where image management would switch to using
heap, whether developer wanted or not, when CONFIG_HEAP_MEM_POOL
gets value greater than zero.
Now when heap is enabled the user can select whether image management
will keep on using static variable, taking static RAM, or will use
heap to allocate the flash image context only when needed.
For this purpose CONFIG_IMG_MGMT_USE_HEAP_FOR_FLASH_IMG_CONTEXT
has been added, which is available when CONFIG_HEAP_MEM_POOL is enabled.

Fixes #44214

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