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/swift/config.go

@@ -50,19 +50,19 @@ func NewConfig() Config {
 }
 
 // ParseConfig parses the string s and extract swift's container name and prefix.
-func ParseConfig(s string) (Config, error) {
+func ParseConfig(s string) (*Config, error) {
 	if !strings.HasPrefix(s, "swift:") {
-		return Config{}, errors.New("invalid URL, expected: swift:container-name:/[prefix]")
+		return nil, errors.New("invalid URL, expected: swift:container-name:/[prefix]")
 	}
 	s = strings.TrimPrefix(s, "swift:")
 
 	container, prefix, _ := strings.Cut(s, ":")
 	if prefix == "" {
-		return Config{}, errors.Errorf("prefix is empty")
+		return nil, errors.Errorf("prefix is empty")
 	}
 
 	if prefix[0] != '/' {
-		return Config{}, errors.Errorf("prefix does not start with slash (/)")
+		return nil, errors.Errorf("prefix does not start with slash (/)")
 	}
 	prefix = prefix[1:]
 
@@ -70,7 +70,7 @@ func ParseConfig(s string) (Config, error) {
 	cfg.Container = container
 	cfg.Prefix = prefix
 
-	return cfg, nil
+	return &cfg, nil
 }
 
 // ApplyEnvironment saves values from the environment to the config.