Fix (#688): Updated JSON tag for ChannelID field in AndroidConfig.Notification

[Surya-7890]

In the try it out example in , we can see that the channelId field in android.notification is in camel case,

{
  message: {
      android: {
          notification: {
              channelId: ""
          }
      }
  }
}

While in the SDK, the json tag for that field is set to the following

ype AndroidNotification struct {
        // ... other fields
	ChannelID             string                        `json:"channel_id,omitempty"`
}

FIx:
Modified the occurances of this fields

firebase-admin-go/messaging/messaging.go

Line 180 in 570427a

ChannelID string `json:"channel_id,omitempty"`

and

firebase-admin-go/messaging/messaging_test.go

Line 227 in 570427a

"channel_id": "channel",