dubbo-sample-async-result example (#15567)

[yashvi1912]

This PR adds a new example module dubbo-sample-async-result under dubbo-samples-advanced to demonstrate asynchronous RPC using CompletableFuture.

Features:

• Demonstrates async result handling using Dubbo
• Uses CompletableFuture for non-blocking calls
• Includes API, Provider, and Consumer modules
• Follows existing sample structure conventions

Structure:

• api: Service interface
• provider: Async service implementation
• consumer: Async invocation example

Verification:

• Successfully built using mvn clean install
• Provider and Consumer run successfully
• Verified async response flow between services

Fixes #15567