Ability to download artifacts from all runs in a folder

This should include parallelisation, so that multiple artifacts are downloaded simultaneously, in order to get the best performance.

Could extend get_artifacts_as_files, e.g.

client.get_artifacts_as_files(run=run_name, path='/tmp', category='output')

or:

client.get_artifacts_as_files(folder=run_name, path='/tmp', category='output')

首先定位现有的 get_artifacts_as_files 实现，以及它用来识别某次运行的构件的 API。确定运行文件夹是如何表示的，然后验证所有匹配的输出构件都会并行下载到请求的路径；当所提议的运行或文件夹调用能够正确处理每次运行和每个类别时，即表示完成。