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/location/location.go

@@ -29,7 +29,7 @@ type parser struct {
 	stripPassword func(string) string
 }
 
-func configToAny[C any](parser func(string) (C, error)) func(string) (interface{}, error) {
+func configToAny[C any](parser func(string) (*C, error)) func(string) (interface{}, error) {
 	return func(s string) (interface{}, error) {
 		return parser(s)
 	}