Allow apiClient to accept Express app instance

[michaelnjuguna]

Problem

Currently, the apiClient plugin only accepts a URL string (or an options object with baseURL).
I would like to pass an Express app instance directly for testing, similar to how Supertest works,
so that we can run tests without starting a server on a real port.

Suggested solution

Allow apiClient to accept an Express app instance and internally handle requests against it.

Use case

This would allow faster tests in CI environments and eliminate port conflicts during testing.

Example:

import app from './app'
apiClient(app) // currently not supported

[thetutlage]

Sure, feel free to send a PR for the same. Maybe we can make it work with Express and the vanilla Node server as well