Add S3 backend support

[AdrianSimionov]

Is your feature request related to a problem? Please describe.
There are not many options on the market to use a sync service to an S3 backend.

Describe the solution you'd like
Since maestral already solved the problem of monitoring the local filesystem, it could be extended to other cloud backends, the most popular probably being S3.

Describe alternatives you've considered
The closest stable app on MacOS I could find is CloudMounter, which supports a lot of backends but will only cache files locally for a period of time, without permanently keeping a local copy.

Additional context
Majority of S3 apps focus on mounting a remote endpoint, not syncing and doing near real time replication.

[arthrod]

I'd love that too!