feature(payments-plugin): PayPal two-step flow implementation

[sonntag-philipp]

Description

This PR introduces a PayPal payments plugin based on the PayPal Orders Api with a 2 step flow as specified in the Vendure documentation.

The main purpose of this plugin is to offer the user a solution where PayPal can be used directly without a step in between.

Documentation will be added by me before this plugin is merged. Just want to make sure I don't have to change it after review feedback of the implementation.

Breaking changes

I am not able to determine a breaking change. The implemented features represent new functionality and should have no effect on existing implementations.

Overview

• Only supports 2 step payments
• Dev server contains ui that can be used to test payments
• A single mutation is added to create PayPal orders
• Integration and unit tests were added and should cover most of the functionality

Screenshots

Checklist

📌 Always:

• [x] I have set a clear title
• [x] My PR is small and contains a single feature
• [x] I have checked my own PR

👍 Most of the time:

• [x] I have added or updated test cases
• [ ] I have updated the README if needed