[SPARK-52742] Support createDataflowGraph

### What changes were proposed in this pull request?

This PR aims to support `createDataflowGraph` API in order to support `Declarative Pipelines` (SPARK-51727) of Apache Spark `4.1.0-preview1`.

### Why are the changes needed?

To support the new feature incrementally.

### Does this PR introduce _any_ user-facing change?

No, this is a new feature.

### How was this patch tested?

Manually run `Apache Spark 4.1.0-preview1` RC1 .

```
$ sbin/start-connect-server.sh
```

Run the newly added unit test.

<img width="1200" alt="Screenshot 2025-07-09 at 19 18 22" src="https://github.com/user-attachments/assets/3084d453-2fc9-45de-873d-0385f84937d6" />

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#208 from dongjoon-hyun/SPARK-52742.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Sources/SparkConnect/SparkConnectClient.swift

@@ -1183,6 +1183,37 @@ public actor SparkConnectClient {
     return try await execute(self.sessionID!, command)
   }
 
+  @discardableResult
+  func createDataflowGraph(
+    _ defaultCatalog: String? = nil,
+    _ defaultDatabase: String? = nil,
+    _ sqlConf: [String: String]? = nil
+  ) async throws -> String {
+    try await withGPRC { client in
+      var graph = Spark_Connect_PipelineCommand.CreateDataflowGraph()
+      if let defaultCatalog {
+        graph.defaultCatalog = defaultCatalog
+      }
+      if let defaultDatabase {
+        graph.defaultDatabase = defaultDatabase
+      }
+      if let sqlConf {
+        graph.sqlConf = sqlConf
+      }
+
+      var pipelineCommand = Spark_Connect_PipelineCommand()
+      pipelineCommand.commandType = .createDataflowGraph(graph)
+
+      var command = Spark_Connect_Command()
+      command.commandType = .pipelineCommand(pipelineCommand)
+
+      let response = try await execute(self.sessionID!, command)
+      let result = response.first!.pipelineCommandResult.createDataflowGraphResult
+
+      return result.dataflowGraphID
+    }
+  }
+
   private enum URIParams {
     static let PARAM_GRPC_MAX_MESSAGE_SIZE = "grpc_max_message_size"
     static let PARAM_SESSION_ID = "session_id"

Tests/SparkConnectTests/SparkConnectClientTests.swift

@@ -96,4 +96,15 @@ struct SparkConnectClientTests {
     }
     await client.stop()
   }
+
+  @Test
+  func createDataflowGraph() async throws {
+    let client = SparkConnectClient(remote: TEST_REMOTE)
+    let response = try await client.connect(UUID().uuidString)
+    if response.sparkVersion.version.starts(with: "4.1") {
+      let dataflowGraphID = try await client.createDataflowGraph()
+      #expect(UUID(uuidString: dataflowGraphID) != nil)
+    }
+    await client.stop()
+  }
 }