use AWS.CredentialProviderChain to resolve credentials

[benconnito]

instead of having to specify credentials resolve credentials using the default chain. this allows an ec2 role to be used and not having to export aws vars in the shell

https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/CredentialProviderChain.html#defaultProviders-property

[salesforce-cla[bot]]

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