backend: let ParseConfig return a Config pointer

In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.

An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.

internal/backend/mem/mem_backend_test.go

@@ -18,8 +18,9 @@ type memConfig struct {
 func newTestSuite() *test.Suite[*memConfig] {
 	return &test.Suite[*memConfig]{
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (*memConfig, error) {
-			return &memConfig{}, nil
+		NewConfig: func() (**memConfig, error) {
+			cfg := &memConfig{}
+			return &cfg, nil
 		},
 
 		// CreateFn is a function that creates a temporary repository for the tests.