Allows Cloudshell to acquire perm from AWS_CONTAINER_CREDENTIALS_FULL v2

[kuettai]

Issue #2323 , if available:

Description of changes:
Added supports for AWS_CONTAINER_CREDENTIALS_FULL_URI in EcsCredentialProvider.php, this will allows aws-php-sdk to works with CloudShell correctly. Permission in Cloudshell to follow IAM User.

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[kuettai]

ATTN: @stobrien89

Previous PR #2324 , changes made to the comment.
Would also solve PR #2249 as per highlighted in PR #2324

[stobrien89]

Hi @kuettai,

Thanks for taking the time to submit the changes. I'll discuss with the team and get back to you as soon as I can. If it's alright with you, I'll close the older PR and focus on this one for now.

[stobrien89]

Implementation looks good- the testing just needs to be refined. Ideally we'd want to have a separate test in EcsCredentialProviderTest that simulates a scenario where we'd be using these two env vars and checks that things like the Authorization header are being set correctly.

[stobrien89]

Conditional approval to run checks

[stobrien89]

Left a comment regarding the uri retrieval logic. We'll need to re-order some things for sake of consistency with other SDKs

[stobrien89]

Left one more comment. Appreciate your patience here. I think we'll be ready for a final review after it's addressed.

[stobrien89]

Pending team review

[stobrien89]

Hi @kuettai and @BenHarris,

Thanks again for your work on this. Tested manually using docker as well as cloudshell and everything works as expected.