Only create aws-sdk DynamoDB db spans when there is a parent span #25801

The aws-sdk integration creates DynamoDB db spans when there is no active span. Each of these spans becomes its own root span and has no context.

In packages/server-utils/src/integrations/aws-sdk/index.ts, create no DynamoDB span when there is no active span. Do not change other services (S3, SQS, SNS, Lambda): a root producer span can be valid for them.

Add a test that checks that an operation without an active span creates no span.