Automatically obtain S3 credentials for a granule with an /s3credentials endpoint

[chuckwondo]

Downloading a granule from S3 that requires credentials takes quite a bit of manual effort (i.e. custom code), but should not require any such effort.

It requires checking the granule's metadata for an /s3credentials endpoint, then obtaining creds via the MAAP.AWS object, then injecting the creds into the default s3 session object (since we cannot supply a custom session object -- see #83), then calling Result.getData on a granule object. Ideally, automatic creds refresh should also be handled.

To make it extremely easy for a maap-py user, all of this should occur automatically, such that when Result.getData is called to download a granule from s3, all of the above should automatically happen for the user, so that no custom code required in such cases.

Ideally, this should make use of the implementation of #83 to simplify things a bit.