Returns AWSCredentials which the caller can use to authorize an AWS request.
Each implementation of AWSCredentialsProvider can chose its own strategy for
loading credentials. For example, an implementation might load credentials
from an existing key management system, or load new credentials when
credentials are rotated.
Forces this credentials provider to refresh its credentials. For many
implementations of credentials provider, this method may simply be a
no-op, such as any credentials provider implementation that vends
static/non-changing credentials. For other implementations that vend
different credentials through out their lifetime, this method should
force the credentials provider to refresh its credentials.