The well know design pattern is called: an API.
It provides a delineation between two sections of code, and describes how those two sections can interact and the reasonable expectations that both sides can have of the other.
This pattern though is very generic.
What you are looking for is a specific kind of API called a Plugin.
The purpose of a plugin is to provide an API for a set of actions, the implementation of those actions provided by code that may as yet not be written, but as it conforms to the API, and discoverability mechanisms can be loaded and plugged in.
The Plugin itself once discovered is probably provided via Dependency Injection to the code in need of its services.
The plugin itself is may be an Adaptor as the API the plugin must provide is probably not the API of the service it is attempting to plugin.
Alternately the plugin may be a Facade hiding a much larger and more capable implementation that has been organised to allow it to be plugged in.
It might be both a facade and an adaptor.
It is also probably that their are several plugin decorators providing additional behaviour or behaviour overrides, such as extra logging, security checking, and fixups for broken third-party implementations.
Perhaps its a nitpick, but you should consider Configuration to be code.
It is code, because it isn't plain data.
Plain data does not change the behaviour of the system. It is simply transported and transformed by the code, it does not change the code paths the code chooses.
On the other hand that configuration file is choosing the backend. Changing the config changes the backend, a change in behaviour, therefor it is code.
