File.zip_file: support custom password

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the new behaviour?

Alternative password can be set in MWDB configuration for files downloaded as zip. The default is "infected".

Closing issues

closes #1092

[yankovs]

can lower memory footprint here I think:

if self.upload_stream is not None:
    self.upload_stream.seek(0)
    for chunk in iter(lambda: self.upload_stream.read(chunk_size), b""):
        zipf.write(chunk)
else:
    with self.open() as fsrc:
        for chunk in iter(lambda: fsrc.read(chunk_size), b""):
            zipf.write(chunk)

[psrok1]

shutil.copyfileobj does exactly that (copying in chunks): https://github.com/python/cpython/blob/main/Lib/shutil.py#L249

[yankovs]

what do you mean by "on the fly"?

[psrok1]

Memory usage improvements have been implemented here: #1108

By "on the fly" I mean zipping file in chunks and sending the zipped part as soon as possible, without waiting for a whole file to be compressed.