feat(example): Allow configuring some parameters with env variables (#663)

Co-authored-by: Andrey Rusakov <andrey.rusakov@camptocamp.com>

example/server/config/config.go

@@ -0,0 +1,40 @@
+package config
+
+import (
+	"os"
+	"strings"
+)
+
+const (
+	// default port for the http server to run
+	DefaultIssuerPort = "9998"
+)
+
+type Config struct {
+	Port        string
+	RedirectURI []string
+	UsersFile   string
+}
+
+// FromEnvVars loads configuration parameters from environment variables.
+// If there is no such variable defined, then use default values.
+func FromEnvVars(defaults *Config) *Config {
+	if defaults == nil {
+		defaults = &Config{}
+	}
+	cfg := &Config{
+		Port:        defaults.Port,
+		RedirectURI: defaults.RedirectURI,
+		UsersFile:   defaults.UsersFile,
+	}
+	if value, ok := os.LookupEnv("PORT"); ok {
+		cfg.Port = value
+	}
+	if value, ok := os.LookupEnv("USERS_FILE"); ok {
+		cfg.UsersFile = value
+	}
+	if value, ok := os.LookupEnv("REDIRECT_URI"); ok {
+		cfg.RedirectURI = strings.Split(value, ",")
+	}
+	return cfg
+}