Add configurable fixed amount for fee

.env_example

@@ -6,3 +6,4 @@ JWT_EXPIRY=604800
 LND_ADDRESS=
 LND_MACAROON_HEX=
 LND_CERT_HEX=
+FIXED_FEE=10

lib/service/config.go

@@ -11,4 +11,5 @@ type Config struct {
 	LNDCertHex     string `envconfig:"LND_CERT_HEX"`
 	CustomName     string `envconfig:"CUSTOM_NAME"`
 	Port           int    `envconfig:"PORT" default:"3000"`
+	FixedFee       int    `envconfig:"FIXED_FEE" default:"10"`
 }