Add ECS credential provider

[kggilmer]

Issue #

#18

Description of changes

• Add ECS credential provider in SDK

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[aajtodd]

Is there any documentation on how to test these?

I know CRT hides all the configuration that would ultimately allow us to setup integration tests. Next best thing would be how to test each provider manually (setup, scenarios, etc).

[kggilmer]

Is there any documentation on how to test these?

I've documented the manual testing I've done for this particular CRT provider in notes here: awslabs/aws-crt-java#389

I am unaware of ways of unit testing these based on how they are implemented in CRT. I've filed an issue to make the ports configurable from Java in this case to allow us to do some testing. Until/if that's implemented I am not aware of a way we can test in CI.

Regarding STS, I have been unable to find documentation that describes the basic happy-path workflow for either assume role or web identity. I have done some manual testing but without knowing what service-side configuration is required, it's not possible to really know if the ad-hoc tests are valid.

The same applies regarding ports and instrumentation for STS as it does to ECS.