Support non-seekable output

[kleisauke]

Description
For writing, it's currently mandatory to provide the ZIP_SOURCE_SEEK_WRITE and ZIP_SOURCE_TELL_WRITE commands in zip_source_callback.

Solution
Allow non-seekable output by making these commands optional. When the output is not seekable one can write a 'streamed' ZIP (by setting bit 3 of the general purpose bit flag) with the CRC-32, compressed size and uncompressed size filled with zero in the local header. The correct values are put in the data descriptor immediately after the compressed data.

Additional context

libzip/lib/zip_close.c

Lines 380 to 381 in b3ac716

	/* as long as we don't support non-seekable output, clear data descriptor bit */
	de->bitflags &= (zip_uint16_t)~ZIP_GPBF_DATA_DESCRIPTOR;

[dillof]

This won't work for modifying existing zip archives (since I don't see a way to replace the original archive with the new one), but for creating new archives it would be a meaningful enhancement.

So we're not opposed to adding this functionality, but it's not high on our priority list. Patches are welcome ;).