feat: process leave encashment via journal entry

[AyshaHakeem]

Previously, Leave Encashment was always processed by creating an Additional Salary on submission, which was later paid through the Salary Slip.

• Add an option to process Leave Encashment via a Payment Entry instead of a Salary Slip.
• Introduced a status field to track payment status.
• Status now updates when paid via Payment Entry or Salary Slip.

Patch to update payment status of existing, submitted leave encashment docs based on whether they have been paid via salary slip

Before:
No status tracking, only Salary Slip-based payment.

After:

Documentation Link: https://docs.frappe.io/hr/leave-encashment

---

This is a backport of pull request #2775 .