Enable a backend.
Mark a backend as enabled, such that the BackendStore will always attempt to load it when BackendStore.load_backends is called. This will not load the backend if it's not currently loaded.
This method is safe to call multiple times concurrently (e.g. an asynchronous call may begin after a previous asynchronous call for the same backend name has begun and before it has finished).
If the backend is disallowed by the FOLKS_BACKENDS_ALLOWED and/or FOLKS_BACKENDS_DISABLED environment variables, this method will store the fact that it should be enabled in future, but will not enable it during this application run.
the name of the backend to enable