Feature/credential provider

[jeskew]

Per the discussion at aws/aws-sdk-js#1483, the homedir locator will use the same logic as the AWS CLI rather than relying solely on os.homedir.

In line with offline discussion, I'm breaking this into a few separate PRs:

• Add a base credential provider package #23 Base credential provider (chain, memoization, credentials -> credential provider converter)
• Feature/credential provider env #24 Environment variable provider (depends on Add a base credential provider package #23)
• Feature/credential provider ini #25 INI file variable provider (depends on Add a base credential provider package #23)
• Feature/credential provider imds #26 IMDS provider (depends on Add a base credential provider package #23)
• Add a default credential provider package for use in Node applications #30 Default node provider chain (depends on Add a base credential provider package #23, Feature/credential provider env #24, Feature/credential provider ini #25, and Feature/credential provider imds #26)

[chrisradek]

So here's an interesting question. Should we move to the next Provider in the chain no matter what the error is (like we do today) or should we stop the chain if a Provider rejects due to bad config (like Python, e.g. Incorrect profile specified)?

I'm leaning towards the latter, since we've had some customers complain about the error messages our chain currently gives. Maybe this is a good first use for the cancellation token?

[jeskew]

Given the way the chain is linked together with .catch handlers, error messages for anything other than the EC2 provider will just get swallowed and replaced with a less useful message.

Rather than a cancellation token, what if the providers always threw a CredentialsError with a tryNextLink boolean property? Bad config errors would set the property to false, unforeseen errors would return undefined, and the next handler in the chain would only be invoked if the property were true.

[chrisradek]

Sure, that sounds reasonable.

[jeskew]

Fully implemented via #23, #24, #25, #26, and #30

[lock]

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