Add support for ListBuckets API

[silvestrpredko]

Is your feature request related to a problem? Please describe. I faced a problem that I couldn't check if the bucket exists or not. Also, it would be nice to get a list of buckets for other purposes.

Describe the solution you'd like Would be nice to have an API where I could pass credentials and region, then it returns a list of buckets. Actually, this request is in S3 API.

Describe alternatives you've considered For now, I have a workaround. Just try to get a list of objects for the bucket. If the request fails bucket doesn't exist. I don't like this.)