Fix per secret overrides for role_arn and role_session_name AWS Secrets Manager integration

What does this MR do?

• This MR fixes the per secret configuration for the AWS Secrets Manager integration as referred to here: https://docs.gitlab.com/ci/secrets/aws_secrets_manager/#per-secret-configuration-overrides
• Prior to this change, the server configuration was taking precedence over the per-secret configuration

Why was this MR needed?

• Customers are unable to leverage the per secret overrides for AWS Secrets Manager for their CI/CD workflows

What's the best way to test this MR?

• Pull down this change and try the per-secret overrides
• Unit test: make simple-test TEST_PKG=./helpers/secrets/resolvers/aws/

What are the relevant issue numbers?

gitlab#581307