Public dataset class for S3A integration tests

Introduction of PublicDatasetTestUtils as proposed previously in some of the ideas for refactoring S3A incrementally. Some of its responsibilities:

• Source of truth for getting URI based on public data set.
• Maybe keep the methods specific to their purpose where possible? We might need s3a://landsat-pds/scene_list.gz specifically for some tests, but other tests may just need a bucket with a bunch of keys.
• Introduce test assumptions about the S3 endpoint or AWS partition. If we’re not looking at 'aws' partition, skip test.

How might we make this generic for non-aws partition S3 or S3API-compatible object stores?

• Ideally allow for future extension to provide some easy ways to override the bucket if tester has an alternative source? I see "fs.s3a.scale.test.csvfile" already has a little bit of this.
• We could have something which takes a path to a hadoop XML config file; we'd have a default resource but the maven build could be pointed at another via a command line property. this file could contain all the settings for a test against a partition or internal s3-compatible store