Credentials issues when running on EC2/ECS instances

[jaytaph]

When running this library on an ECS or EC2 instance, the credentials are normally provided through the ec2 meta data and do not need to be set explicitly. However, it seems that this library does not support this: it needs an explicit KEY and SECRET and leaving it blank will result in invalid calls.

Removing the 'credentials' key in the $config in CognitoAuthServiceProvider works but breaks valid key/secret credentials.

[oheck]

This package is a POC / WIP. Feel free to fork and modify the behaviour accordingly.