feat: add generic method to support custom extensions

Author: hasam-hasam1_cba

src/client/client.ts

@@ -347,6 +347,19 @@ export class A2AClient {
     return this._postRpcRequest<TaskIdParams, CancelTaskResponse>("tasks/cancel", params);
   }
 
+  /**
+   * @template TExtensionParams The type of parameters for the custom extension method.
+   * @template TExtensionResponse The type of response expected from the custom extension method. 
+   * This should extend JSONRPCResponse. This ensures the extension response is still a valid A2A response.
+   * @param method Custom JSON-RPC method defined in the AgentCard's extensions.
+   * @param params Extension paramters defined in the AgentCard's extensions.
+   * @returns A Promise that resolves to the RPC response.
+   */
+  public async callExtensionMethod<TExtensionParams, TExtensionResponse extends JSONRPCResponse>(method: string, params: TExtensionParams) {
+    return this._postRpcRequest<TExtensionParams, TExtensionResponse>(method, params);
+  }
+
+
   /**
    * Resubscribes to a task's event stream using Server-Sent Events (SSE).
    * This is used if a previous SSE connection for an active task was broken.