There may be others, but here is one plugin that will take care of things for you:
https://wordpress.org/plugins/amazon-s3-and-cloudfront/
Though you need to use the Pay version to get existing Images moved to Amazon S3.
For other possibilities, try this search:
https://wordpress.org/plugins/search.php?q=amazon+s3
Assuming that you are considering other Object Storage solutions, this will likely be of interest:
https://wordpress.org/plugins/wp-stateless/
Here is the search that may lead to more of this type of plugin:
https://wordpress.org/plugins/search.php?q=cloud+media+library
I am not really used to Google Cloud Storage, but I think I will give it a try, since at this moment I don’t want to pay for a plugin.
First impressions after trying:
Wp Stateless seems to be still in early development because it doesn’t work as it should in some cases. For example, it doesn’t rewrite the URL of post images when they are linked to the original file (when you can click them and open the file from the server) but does it properly when they’re not linked (when you can’t click them). I had to manually change those URL to make it work.
I want to clarify that when I said “I don’t want to pay for a plugin” I meant for this purpose.
