Read credentials from AWS_CREDENTIAL_FILE environment variable

It would be nice if credentials could also be read from a credential file specified by an environment variable, like AWS_CREDENTIAL_FILE, as is available in some other AWS toolkits. We use a variety of toolkits, and prefer the credential file method.

This PR patches AWS::Core::CredentialProviders::ENVProvider to also read from {@prefix}_CREDENTIAL_FILE, and merge those values in. I have another branch that implements a new ENVCredentialFileProvider class, not sure which is a better approach?