Update simulateTransaction rpc handling of return_data, and update docs (#24355)

* Stringify return_data program_id; also camel-case all fields

* Update simulateTransaction json-rpc docs

* Base64-encode return data in simulation RPC responses

client/src/rpc_response.rs

@@ -348,7 +348,29 @@ pub struct RpcSimulateTransactionResult {
     pub logs: Option<Vec<String>>,
     pub accounts: Option<Vec<Option<UiAccount>>>,
     pub units_consumed: Option<u64>,
-    pub return_data: Option<TransactionReturnData>,
+    pub return_data: Option<RpcTransactionReturnData>,
+}
+
+#[derive(Serialize, Deserialize, Clone, Debug)]
+#[serde(rename_all = "camelCase")]
+pub struct RpcTransactionReturnData {
+    pub program_id: String,
+    pub data: (String, ReturnDataEncoding),
+}
+
+impl From<TransactionReturnData> for RpcTransactionReturnData {
+    fn from(return_data: TransactionReturnData) -> Self {
+        Self {
+            program_id: return_data.program_id.to_string(),
+            data: (base64::encode(return_data.data), ReturnDataEncoding::Base64),
+        }
+    }
+}
+
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub enum ReturnDataEncoding {
+    Base64,
 }
 
 #[derive(Serialize, Deserialize, Clone, Debug)]