CredentialsProperties encapsulates reading AWS credentials from configuration.

This is a wrapper class, designed to allow us to recognize and retrieve credentials from the environment,
config files, or other sources.
Added FileLocator and implementations.

FileLocator, and its implementations, are a common wrapper around local Files, classpath locations,
bucket/keyname pairs in S3, and static byte arrays (for testing).  This is a way (also serializable)
for naming the location of parquet files and indices in different systems.