refactor: clarify architecture, implement PayZen & implement Wompi

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ERP Companion: frappe/erpnext#40845 🟢

Frappe Companion: frappe/frappe#25952 ✔️ & frappe/frappe#26324 ✔️ & frappe/frappe#26637 ✔️

closes #52

Rendered

@s-aga-r @ankush please let me know what you think of this direction.

• Merge Frappe Hook update feat: add new payment processing hooks frappe#25952
  • Ensure it is fully backwards compatible
• Merge ERPNext preparation feat: enable a new payment controller erpnext#40845
  • Implement ERPNext dispatcher for new and old version gateways on Payment Request
  • Ensure it is fully backwards compatible
• Revise docs (now that the implementation is almost finished) - special note: how to implemeten payment sessions for webshop in the future
• cleanup old payzen implementation
• cleanup commit history
• add wompi implementation
• write unit tests (new controller, wompi & payzen)
• Investigate impact of frappe/frappe@59ca074 cc @akhilnarang (removal of response.raise_for_status())

* This implementation is quite eagerly typed to aid with IDE support when implementing a new gateway.

** Walk through screenshots: #53 (comment)

Overview

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Some Noetes to self

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