Make it possible to connect to a dummy API to perform tests

[gerben86]

What problem does this feature solve?

Make it possible to connect to a test-api, so we can do test-shipments etc.
Especially with debugging issues where MyParcel and another plugin/extension are interfering each other it could be easy to have some fake replies from an endpoint but also to do the order-status-changes without creating real shipments in MyParcel.

Thinking aloud:
It would even be nicer if there would be 'static' replies, based on the postalcode.
By examle:
5500AA -> returns a 500 from the MyParcel-Test-API
4401AA -> returns a 401 from the MyParcel-Test-API
2200AA -> returns a 200 from the MyParcel-Test-API
etc.

What should the solution look like?

Just like the Mollie-plugin:

• Switch between Live and Test
• Store the Live and Test api-keys

Relevant log output

No response

Additional context

No response