Credential providers should be chainable.

[jasdel]

The V2 SDK should allow credential providers to be constructed in a chain. The chain of providers should be refershable. Meaning that parent providers should refreshable at the request of a child provider.

[jasdel]

This is a v2 port of the v1 feature

• aws/session: Add support for assuming role via Web Identity Tokens aws-sdk-go#2667
• Looking for credentials file when env vars configured causes error aws-sdk-go#2455
• aws/session: SDK gives precedence to AWS_PROFILE over env credentials. aws-sdk-go#2727

[skmcgrail]

Now support as part of #488