HDDS-14010. [Recon] Endpoint to retrieve pending deletion metrics from DataNodes, SCM, and OM.

[priyeshkaratha]

What changes were proposed in this pull request?

This pull request introduces a new REST API endpoint /pendingDeletion in the Recon service to provide comprehensive visibility into pending block deletions across different components of the Ozone system. The endpoint allows administrators to monitor the status of pending deletions from three key components:

1. DataNodes (DN): Pending blocks marked for deletion on individual datanodes
2. Storage Container Manager (SCM): Aggregated deleted block transaction summary
3. Ozone Manager (OM): Pending deletion information from the object store manager

New REST Endpoint: PendingDeletionEndpoint

• Routes to /pendingDeletion with support for a required component query parameter
• Supports three component options: dn, scm, om
• Implements asynchronous metric collection for DataNode metrics with proper status handling
• Returns appropriate HTTP status codes (202 Accepted for ongoing tasks, 200 OK for completed results, 400 Bad Request for invalid parameters)

What is the link to the Apache JIRA

HDDS-14010

How was this patch tested?

Added integration test cases