fix(config): add the option of a default provider

mailout.yml.sample

@@ -5,6 +5,7 @@ postgres:
   user: postgres
   password: postgres123
   sslmode: disable
+defaultProvider: ovh
 providers:
   ovh:
     application_key: <CHANGEME>

pkg/config/config.go

@@ -16,7 +16,8 @@ type Config struct {
 		Database string `yaml:"database"`
 		SSLMode  string `yaml:"sslmode"`
 	} `yaml:"postgres"`
-	Providers struct {
+	DefaultProvider string `yaml:"defaultProvider"`
+	Providers       struct {
 		OVH *providerConfigs.OVHConfig `yaml:"ovh"`
 	} `yaml:"providers"`
 }