S3A: Add ability to plug in any AWSCredentialsProvider

Although S3A currently has built-in support for org.apache.hadoop.fs.s3a.BasicAWSCredentialsProvider, com.amazonaws.auth.InstanceProfileCredentialsProvider, and org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider, it does not support any other credentials provider that implements the com.amazonaws.auth.AWSCredentialsProvider interface. Supporting the ability to plug in any com.amazonaws.auth.AWSCredentialsProvider instance will expand the options for S3 credentials, such as:

• temporary credentials from STS, e.g. via com.amazonaws.auth.STSSessionCredentialsProvider
• IAM role-based credentials, e.g. via com.amazonaws.auth.STSAssumeRoleSessionCredentialsProvider
• a custom credentials provider that satisfies one's own needs, e.g. bucket-specific credentials, user-specific credentials, etc.

To support this, we can add a configuration for the fully qualified class name of a credentials provider, to be loaded by S3AFileSystem.initialize(URI, Configuration).

The configured credentials provider should implement com.amazonaws.auth.AWSCredentialsProvider and have a constructor that accepts (URI uri, Configuration conf).