feat: Pricing Rule

[akshayitzme]

This PR adds the functionality of Pricing Rules

Checklist

• [x] Create & Update Schemas for Pricing Rule
• [x] Create & Update Models for Pricing Rule
• [x] Pricing Rule Functionality
• [x] Pricing Rule Feature in Point of Sale
• [x] Tests
• [x] Documentation: https://github.com/frappe/books_docs/pull/29