Add ListBuckets endpoint

Closes #1051 (closed)

The settings for the Minio client are weird. There cannot be a scheme in the endpoint, unlike boto3, and the region is irrelevant. Since we are working with multiple buckets, they could be on different regions, so the client just detects the region by itself when listing a specific bucket.

This results in the following settings:

s3:
  endpoint: https://somewhere.com  # optional, defaults to AWS
  access_key_id: blah
  secret_access_key: blah
  region: blah
ingest:
  endpoint: somewhere.com  # required
  access_key_id: blah
  secret_access_key: blah