Update and integrate price updater

PriceUpdater:
    - Pass context so that it can be canceled during an update loop
    - Define APITypes to make it explicit which API we are using

config/config.go

@@ -97,6 +97,11 @@ type Coordinator struct {
 
 // Node is the hermez node configuration.
 type Node struct {
+	PriceUpdater struct {
+		Interval Duration `valudate:"required"`
+		URL      string   `valudate:"required"`
+		Type     string   `valudate:"required"`
+	} `validate:"required"`
 	StateDB struct {
 		Path string
 	} `validate:"required"`