Refactor Sender interface, split SendEvents method

[Dimedrolity]

Overview

We have Sender interface with SendEvents method.

// Sender interface for implementing specified contact type sender
type Sender interface {
	SendEvents(events NotificationEvents, contact ContactData, trigger TriggerData, plot [][]byte, throttled bool) error
	Init(senderSettings map[string]string, logger Logger, location *time.Location, dateTimeFormat string) error
}

SendEvents is public method of Sender interface, so it is part of public API.

Problem

Now public API is doesn't allow to test that message is built correctly, because SendEvents has only error in return params. How to check that message is built correctly?

Quick solution (current solution)

Now we have test of internal methods buildMessage and sendMessage.

Right solution (TODO)

Need to separate interface method SendEvents to two methods - BuildMessage and SendMessage - to test methods separately as public API. Also method doesn't fit SRP, because it has two reasons to change, for example when need to change message format and need to change delivery of message.

[Dimedrolity]

After that we can separate to 2 interfaces, Sender and MessageBuilder. Also we can extract Message struct. Just thoughts