feat: add virtual threads support for gRPC server

- Add VirtualThreadGrpcServerCustomizer to enable virtual threads when spring.threads.virtual.enabled=true
- Add comprehensive tests for virtual threads functionality
- Update documentation with virtual threads configuration examples
- Add example configuration in application-grpc-server-boot-starter-example.yaml

Closes #118

Author: DanielLiu1123

grpc-boot-autoconfigure/grpc-server-boot-autoconfigure/src/main/resources/application-grpc-server-boot-starter-example.yaml

@@ -1,3 +1,8 @@
+spring:
+  threads:
+    virtual:
+      enabled: true                             # enable virtual threads (Java 21+)
+
 grpc:
   server:
     enabled: true                               # whether to enable grpc server

website/docs/20-server/10-autoconfiguration.mdx

@@ -145,6 +145,23 @@ grpc:
       enabled: true
 ```
 
+## Virtual Threads Support
+
+Starting from version [3.5.3.2](https://github.com/DanielLiu1123/grpc-starter/releases/tag/v3.5.3.2), gRPC server supports virtual threads when using Java 21+.
+
+When `spring.threads.virtual.enabled` is set to `true`, the gRPC server will automatically use virtual threads to handle requests, providing better performance for I/O-bound operations.
+
+```yaml
+spring:
+  threads:
+    virtual:
+      enabled: true
+```
+
+:::info
+Virtual threads are only available in Java 21+. This feature will be ignored on older Java versions.
+:::
+
 ## Configurations
 
 Full configuration is available in the [`GrpcServerProperties`](https://github.com/DanielLiu1123/grpc-starter/blob/main/grpc-boot-autoconfigure/grpc-server-boot-autoconfigure/src/main/java/grpcstarter/server/GrpcServerProperties.java).