Implement AWS-S3 cloud storage backend

[constantinpape]

The new C++ API allows to implement other backends than the filesystem.
The first to implement is AWS-S3. There is a mock-up implementation for this already.
In order to actually implement it, the functionality in the s3 namespace needs to be implemented using the aws-c++-sdk. For reference, see the implementation in namespace filesystem.

Any help with this is welcome, including WIP PRs that only implement parts of this.
I think good first steps to implement this are:

• Implement the file handle
• Implement read and write attributes.
• Make some test data available with n5-java-aws or zarr-python.

[constantinpape]

I started to implement this in #139.