tests/functional: add a module for handling asset download & caching

tests/functional: add a module for handling asset download & caching

The 'Asset' class is a simple module that declares a downloadable
asset that can be cached locally. Downloads are stored in the user's
home dir at ~/.cache/qemu/download, using a sha256 sum of the URL.

[thuth: Drop sha1 support, use hash on file content for naming instead of URL,
        add the possibility to specify the cache dir via environment variable]

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240830133841.142644-15-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>