Bug: [FEATURE]: [PayU] Use `connector_response_reference_id` as reference to merchant



### :memo: Feature Description

- Reference id are used to map transactions in the connector’s dashboard.
- Hyperswitch manages several reference ids, such as `payment_id`, `attempt_id`, and `connector_transaction_id` for a single transaction. 
- However, merchants may encounter uncertainty when determining which ID to utilize in the connector dashboard to identify the payment.

### :hammer: Possible Implementation

- When we receive a response from the connector for the payment, we deserialize it and populate the `response` field in RouterData. 
- For the `TransactionResponse` type, we must fill the `connector_response_reference_id` with a corresponding reference id for the merchant to identify the transaction. 
- One might need to have exposure to api docs of the connector for which it is being implemented to decide what to fill in connector_response_reference_id.
- You can check this PR for further reference https://github.com/juspay/hyperswitch/pull/1735

🔖 Note: All the changes needed should be contained within `hyperswitch/crates/router/src/connector/`


### :package: Have you spent some time checking if this feature request has been raised before?

- [X] I checked and didn't find a similar issue

### :package: Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

###  :sparkles: Are you willing to submit a PR?

diff --git a/crates/router/src/connector/payu/transformers.rs b/crates/router/src/connector/payu/transformers.rs
index c43e59ac76c..9a2e14215c7 100644
--- a/crates/router/src/connector/payu/transformers.rs
+++ b/crates/router/src/connector/payu/transformers.rs
@@ -194,12 +194,17 @@ impl<F, T>
         Ok(Self {
             status: enums::AttemptStatus::from(item.response.status.status_code),
             response: Ok(types::PaymentsResponseData::TransactionResponse {
-                resource_id: types::ResponseId::ConnectorTransactionId(item.response.order_id),
+                resource_id: types::ResponseId::ConnectorTransactionId(
+                    item.response.order_id.clone(),
+                ),
                 redirection_data: None,
                 mandate_reference: None,
                 connector_metadata: None,
                 network_txn_id: None,
-                connector_response_reference_id: None,
+                connector_response_reference_id: item
+                    .response
+                    .ext_order_id
+                    .or(Some(item.response.order_id)),
             }),
             amount_captured: None,
             ..item.data
@@ -326,12 +331,17 @@ impl<F, T>
         Ok(Self {
             status: enums::AttemptStatus::from(item.response.status.status_code.clone()),
             response: Ok(types::PaymentsResponseData::TransactionResponse {
-                resource_id: types::ResponseId::ConnectorTransactionId(item.response.order_id),
+                resource_id: types::ResponseId::ConnectorTransactionId(
+                    item.response.order_id.clone(),
+                ),
                 redirection_data: None,
                 mandate_reference: None,
                 connector_metadata: None,
                 network_txn_id: None,
-                connector_response_reference_id: None,
+                connector_response_reference_id: item
+                    .response
+                    .ext_order_id
+                    .or(Some(item.response.order_id)),
             }),
             amount_captured: None,
             ..item.data
@@ -461,7 +471,10 @@ impl<F, T>
                 mandate_reference: None,
                 connector_metadata: None,
                 network_txn_id: None,
-                connector_response_reference_id: None,
+                connector_response_reference_id: order
+                    .ext_order_id
+                    .clone()
+                    .or(Some(order.order_id.clone())),
             }),
             amount_captured: Some(
                 order

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
Fixes #2344 


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
Fixes #2344 


## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable
