Update devcontainer.json and python.yml, add new payment method request and response models, update VERSION, and update __version__ in __init__.py

[fcarrero]

• Update the image version in devcontainer.json to "mcr.microsoft.com/devcontainers/python:0-3.8"
• Update the python-version matrix in python.yml to include "3.13"
• Add new payment method request model file: conekta/models/payment_method_general_request.py
• Add new payment method response model file: conekta/models/payment_method_token_request.py
• Update the VERSION file from "7.5.0" to "7.9.0"
• Update the version variable in init.py from "6.0.3" to "6.0.4"