Adds GraphQL client for Payments Apps

[mkevinosullivan]

WHY are these changes introduced?

Adds client to support /payments_apps/api endpoint. See https://shopify.dev/api/payments-apps/

Port of PR #454.

WHAT is this pull request doing?

Adds a GraphQL client for supporting the Payments Apps endpoint, similar to Storefront Client.

Type of change

• [ ] Patch: Bug (non-breaking change which fixes an issue)
• [x] Minor: New feature (non-breaking change which adds functionality)
• [ ] Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [ ] I have added a changelog entry, prefixed by the type of change noted above
• [x] I have added/updated tests for this change
• [ ] I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[JTenges]

Hi, any updates on this? Looks like it's been approved, does it just need to be rebased? It would be really helpful for me as I'm working on a payments app.