Add support for MoEngage Datacenter 04 (api-04.moengage.com)

[yugalbfleek]

Issue Title

Feature Request: Add support for MoEngage Datacenter 04 (api-04.moengage.com)

Issue Body

## Summary

Currently, RudderStack's MoEngage destination only supports three datacenters:
- US → api-01.moengage.com
- EU → api-02.moengage.com  
- IND → api-03.moengage.com

MoEngage has a fourth datacenter (Datacenter 04) that uses `api-04.moengage.com`, but there's no way to configure this in RudderStack.

## Use Case

We need to route events to MoEngage's Datacenter 04 endpoint for compliance or performance reasons. Currently, users with accounts on datacenter-04 cannot use RudderStack's MoEngage destination.

## Current Behavior

1. **Transformer Code**: The region mapping in `src/v0/destinations/moengage/config.js` only includes US, EU, and IND
2. **UI Configuration**: The destination configuration UI only shows three region options (US, EU, IND)
3. **Error**: Setting region to "DC04" or any other value results in a configuration error

## Expected Behavior

1. Add "DC04" or "Datacenter 04" as a selectable region option in the UI
2. Support routing to `api-04.moengage.com` when DC04 is selected
3. Maintain backward compatibility with existing US/EU/IND configurations

## Proposed Solution

### Option 1: Add DC04 as a Standard Region
- Add DC04 to the endpoints mapping in transformer code
- Update destination definition schema to include DC04 in UI dropdown
- This would be the cleanest approach for users

### Option 2: Support Custom Datacenter
- Add a "Custom Datacenter" field that allows users to specify any datacenter number (1-6)
- More flexible but requires UI changes

### Option 3: Environment Variable Override
- Allow `MOENGAGE_API_ENDPOINT` environment variable to override the endpoint
- Less user-friendly but easier to implement

## Technical Details

**Files that need changes:**
- `rudder-transformer/src/v0/destinations/moengage/config.js` - Add DC04 endpoints
- Destination definition schema (in `rudder-server` or `config-generator`) - Add DC04 to UI options

**Current code structure:**
```javascript
const endpoints = {
  US: { /* api-01.moengage.com */ },
  EU: { /* api-02.moengage.com */ },
  IND: { /* api-03.moengage.com */ }
};

Proposed addition:

DC04: {
  identify: `https://api-04.moengage.com/v1/${endpointPaths.identify}/`,
  track: `https://api-04.moengage.com/v1/${endpointPaths.track}/`,
  device: `https://api-04.moengage.com/v1/${endpointPaths.device}/`,
  alias: `https://api-04.moengage.com/v1/${endpointPaths.alias}?app_id=`,
}

Possible Repositories impacted

Primary: rudderlabs/rudder-transformer
Secondary (if UI-related): rudderlabs/rudder-server or rudderlabs/config-generator

Questions

1. Is there a reason DC04 was not included initially? (e.g., not available at the time)
2. Would the team be open to a PR adding DC04 support?
3. Which repository should I submit the PR to? (transformer, server, or both?)

Additional Context

• MoEngage documentation confirms datacenter-04 exists: https://help.moengage.com/hc/en-us/articles/360057030512-Data-Centers-in-MoEngage
• We've already implemented a workaround by forking the transformer, but would prefer to use the official version
• This affects users who have MoEngage accounts on datacenter-04

[contributor-support]

Thanks for opening this issue! We'll get back to you shortly. If it is a bug, please make sure to add steps to reproduce the issue.

[github-actions]

This issue is considered to be stale. It has been open for 30 days with no further activity thus it is going to be closed in 7 days. To avoid such a case please consider removing the stale label manually or add a comment to the issue.