Re-add support for STS credential provider for S3

[xokdvium]

Describe the bug

The recent migration away from aws-sdk-cpp (cc @lovesegfault) has most likely regressed STS credentials handling (judging from me poking at aws-crt-cpp code) added in #12646. STS is not part of the default credentials chain:

            /**
             * Configuration options for a provider that implements a cached provider chain
             * based on the AWS SDK defaults:
             *
             *   Cache-Of(Environment -> Profile -> IMDS)
             */
            struct AWS_CRT_CPP_API CredentialsProviderChainDefaultConfig

Aws::Crt::Auth::CredentialsProviderSTSConfig needs special handling and reading of the config to fish out the RoleArn.

The code for STS provider factory is at https://github.com/awslabs/aws-crt-cpp/blob/aa37a25c9a4db2f24ad44f186bc5726057a21e0a/source/auth/Credentials.cpp#L451C72-L489.

Steps To Reproduce

Don't have access to AWS infra to make a reproducer for this.

Expected behavior

Metadata

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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