libsparse: add callback output file type

Add a new output file subclass that will call a callback for
each block as it is written.  Will be used to measure the space
used by each sparse block to allow resparsing files.

Also add sparse_file_callback, which will write out a sparse
file by calling the provided write function.

Change-Id: I18707bd9c357b68da319cc07982e93d1c2b2bee2