New endpoint ListBuckets

We need a new endpoint named ListBuckets that would use static credentials from new settings (and different from the AWS ones):

• INGEST_AWS_ENDPOINT, from ingest.endpoint in yaml
• INGEST_AWS_ACCESS_KEY, from ingest.access_key_id in yaml
• INGEST_AWS_SECRET_KEY, from ingest.secret_access_key in yaml
• INGEST_AWS_REGION, from ingest.region in yaml

The endpoint would list available buckets using minio client list_buckets.

The bucket serializer would simply expose the bucket name. Buckets would be sorted alphabetically.

Bonus: If list_buckets provide the number of objects without extra queries, add it. Otherwise, forget it.