Plugins API: Get/Create Funnels

[aerosol]

Changes

This PR implements basic get/create operations for Funnels via the Plugins API, targeting WooCommerce integrations via our official WordPress plugin. The extension is available only in EE and only for users whose plan setup allows Funnel usage. No deletions were implemented just yet, as they're likely unnecessary at this point. First pass - needs to go live so I can work with @Dan0sz on any readjustments needed.

Tests

• [x] Automated tests have been added
• [ ] This PR does not require tests

Changelog

• [ ] Entry has been added to changelog
• [x] This PR does not make a user-facing change

Documentation

• [ ] Docs have been updated
• [x] This change does not need a documentation update

Dark mode

• [ ] The UI has been tested both in dark and light mode
• [x] This PR does not change the UI