Resolves #19790, Provides Support for IAM Credentials

Includes support for either leveraging environment variables
passed to the PHP runtime or IAM instance profile present
on the host being used. The default and first choice is
still the parameter file as documented.

See also: https://docs.aws.amazon.com/sdk-for-php/v3/developer-guide/guide_credentials_provider.html#chaining-providers



Signed-off-by: Stephen Cuppett <steve@cuppett.com>