Support for systemd credentials

Hi! It'd be nice for webmentiond to support the concept of "[credentials](https://systemd.io/CREDENTIALS/)" introduced by systemd. In short, it's a safer alternative to environment variables for configuring services :) The way it works from the point of view of webmentiond is that instead of reading stuff from the environment it reads it from files located in a directory pointed by the `CREDENTIALS_DIRECTORY` environment variable (automatically set up by systemd or whatever other service manager). Here's a simple example of how it could be implemented (note that credential names are namespaced by default): ```go var credentialsDirectory string = os.Getenv("CREDENTIALS_DIRECTORY") func getCredential(name string) string { name = "org.webmentiond." + name credential, err := os.ReadFile(path.Join(credentialsDirectory, name)) if err != nil { log.Fatal(err) } return strings.TrimSpace(string(credential)) } func main() { if credentialsDirectory == "" { log.Fatal("You must specify the CREDENTIALS_DIRECTORY environment variable") } mailHost := getCredential("mail.host") mailUser := getCredential("mail.user") mailPassword := getCredential("mail.password") // ... } ``` Bye!