Support s3:// protocol

**Describe the problem/challenge you have**

Need to fetch artifacts from AWS S3 buckets.

We, and our vendors, distribute artifacts with S3. Some buckets are secured so can't be accessed anonymously via the `https://` path.

**Describe the solution you'd like**

Support `s3://` like this:

```yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
- path: vendor
contents:
- path: my-package
s3:
url: s3://bucket-name/packages/my-package.tgz
```

It should use the AWS SDK, such that it automatically uses the authentication from the environment (variables, credential file, credential process, instance metadata, ...).

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No files, tests, or entry points are named. Start by locating vendir's existing content URL handling and AWS SDK usage, then verify that an s3:// URL can use the stated environment and metadata credentials; done means the YAML example fetches the artifact from a secured S3 bucket.

Written by the indexing model from the issue text.