[FEATURE] support for parallel downloads

[Mrc527]

Use case
As a user, I want to download the entire content of my iCloud account to have a local backup.
This means, as I own a 2TB account, I have to potentially download a lot of files and pictures. Unfortunately, this tool executes the file download sequentially which causes the sync job to last weeks.

Describe the solution you'd like
I would like to enable parallel downloads of files. This would shorten the script run time and allow a more frequent sync.

Describe alternatives you've considered
I could think of having different folders with different synchronisation times, to allow frequent updates for folders with frequent changes and sporadic updates for folders I use less.

But this solution is very cumbersome and forces me to decide beforehand which folders I want to keep synchronised more and less often.

[mandarons]

asyncio is another option to use. But this will need icloudpy to support asyncio first. Here is the issue in icloudpy to track: mandarons/icloudpy#10