fix(server-core): Support config dict return from externalDriverFactory

[UgoM]

It is my first PR on your repo, so don’t hesitate to help me improve it if I’ve done things wrong.

Check List

• Tests have been run in packages where changes have been made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue

Fixes #10317

Description

external_driver_factory in Python (cube.py) fails because it expects a BaseDriver instance but Python can only return config dicts. This works fine for driver_factory which has conversion logic in resolveDriver(), but externalDriverFactory was missing the same treatment.

This PR adds the same config-to-driver conversion pattern used by driverFactory:

if (isDriver(val)) {
  driver = val;
} else if (val.type && typeof val.type === 'string') {
  const { type, ...rest } = val;
  driver = CubejsServerCore.createDriver(type, rest);
}

Now both factories behave consistently and Python users can use external_driver_factory with config dicts.