[WIP] Add support for Task Roles when running on ECS or CodeBuild

[jekh]

This PR is intended to address #8746. It adds the RemoteCredProvider from the AWS SDK if the AWS_CONTAINER_CREDENTIALS_RELATIVE_URI environment variable is set. The RemoteCredProvider uses the value of that environment variable and appends it to the hard-coded URL http://169.254.170.2, and uses the resulting URL to obtain Task Role credentials.

A few questions about the PR:

1. I'm not sure what Handlers to provide to the RemoteCredProvider, so I went with the SDK defaults. Is this the right pattern to follow for adding this provider?
2. Testing - This is quite difficult to unit test, as the SDK is hard-coded to use 169.254.170.2 for container credentials, making it difficult to mock. Any suggestions?
3. (Somewhat tangential) What's the rationale behind a custom provider chain, rather than allowing the AWS SDK to find credentials using its default search path (at least when credentials aren't specified directly in TF)?

[blaines]

Item 3 is a good point because I expect Terraform to behave similarly to a current aws-sdk

[jekh]

I'm going to close this PR as I've opened a PR against the AWS provider itself, #1425.

[ghost]

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