🚀 Bitbucket-Pull-Requests: Add Bitbucket Cloud support

### Workspace

bitbucket-pull-requests

### 🔖 Feature description

## Feature request: Bitbucket Cloud support

### Current behavior
The Bitbucket Pull Requests plugin currently only supports Bitbucket Data Center. Bitbucket Cloud uses a different REST API to fetch pull requests, documented here:
https://developer.atlassian.com/cloud/bitbucket/rest/api-group-pullrequests/#api-repositories-workspace-repo-slug-pullrequests-get

As a result, teams using Bitbucket Cloud cannot use this plugin to view pull requests in Backstage.

Many organizations use Bitbucket Cloud instead of Data Center. Supporting Cloud would broaden the plugin's usability and allow more teams to integrate pull requests into their Backstage instance.

### 🎤 Context

Currently, the Bitbucket Pull Requests plugin only works with Bitbucket Data Center. Many organizations, however, use Bitbucket Cloud, which exposes a different API for fetching pull requests: [Bitbucket Cloud API Docs](https://developer.atlassian.com/cloud/bitbucket/rest/api-group-pullrequests/#api-repositories-workspace-repo-slug-pullrequests-get)

Without support for Cloud, teams cannot view their pull requests in Backstage or use features like filtering by status (OPEN, MERGED, DECLINED) or seeing build status.

Companies migrating from Data Center to Cloud can continue using the plugin without losing PR visibility.

### ✌️ Possible Implementation

- Detect Bitbucket Cloud vs Data Center via configuration in app-config
- Implement a separate API client for Bitbucket Cloud using the documented REST endpoints
- Reuse existing UI components where possible
- Keep backward compatibility for Data Center users

### 👀 Have you spent some time to check if this feature request has been raised before?

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

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/backstage/community-plugins/blob/main/CODE_OF_CONDUCT.md)

### Are you willing to submit PR?

Yes I am willing to submit a PR!

Start by reading the existing Bitbucket Pull Requests plugin and its app-config integration, then compare its Data Center behavior with the linked Bitbucket Cloud pull-request API documentation. Define how Cloud is detected and how a separate client can coexist with the current implementation. Done means Cloud pull requests are visible with status filtering and build status while Data Center support remains compatible.

Written by the indexing model from the issue text.