[RFC] Add support for AWS Standard Credential Provider Chain

[AdamTylerLynch]

Recommend implementing the standard credential provider chain as per AWS SDK standards. The providers have an order of precedence, and support refresh tokens for federation, operating in containers, and EC2 assume role.

Adding these would provide consistent experiences across runtime environment, and provide the ability to leverage AWS-lite for specific parts of an application (strangler pattern) without having to change/amend the application’s credential provider.

https://docs.aws.amazon.com/sdkref/latest/guide/standardized-credentials.html

[ryanblock]

Stoked for this issue! General thoughts:

• Where feasible and ergonomic for devs, aws-lite aims to adopt the conventions set by the AWS SDK
• As of today, I believe we have support for two steps in the chain: access keys (of course) and process credential provider
• I think it probably makes sense to eventually support any steps in the credential provider chain that are relevant to the aws-lite use case
• Within this context, we may generally break with existing conventions anywhere that has a meaningful impact on initialization or per-request performance; such cases, if supported, would likely be explicitly opt-in

Does AWS have any information / telemetry about the frequency of usage in the provider chain? What steps would be the priority?

Finally: PRs welcome! :)

[ryanblock]

Bump @AdamTylerLynch!

[mhart]

Back in the day I created https://github.com/mhart/awscred – but I imagine there are more credential sources these days