Fix credential provider chain for aws_request_signing filter

[freedomljc]

It's to fix #45643: AWS AssumeRole provider hangs indefinitely when using WebIdentity source credentials in EKS

Here's the root Cause:
To fetch the base credentials needed to sign the STS AssumeRole request, createAssumeRoleCredentialsProvider instantiates an inner CommonCredentialsProviderChain. However, setupSubscriptions() is never called on this newly created inner chain.
Because subscriptions are never set up, the inner chain fails to register its CredentialSubscriberCallbacks with any underlying asynchronous metadata providers (like WebIdentityCredentialsProvider or InstanceProfileCredentialsProvider). When the background metadata fetch eventually succeeds (e.g., STS successfully exchanges the EKS pod's OIDC token for web identity credentials), the base provider fires onCredentialUpdate(). But because there are no registered subscribers, the inner chain is never notified.
Consequently, the pending callbacks attached by the SigV4SignerImpl (via addCallbackIfChainCredentialsPending) are never executed, and the outbound AWS request is left paused indefinitely waiting for credentials to resolve.

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Risk Level: low
Testing: Have validated the fix: After applying the change, the aws_request_sigining filter would be able to fetch aws credentials.
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