feat: create mailer

[mimoham24]

Overview

What I've done

What I haven't done

How I tested

Unit testing:

• encodContent()
• encodeMessage()

MailHog:

• input

SendMail([]gateway.Contact{{
		Email: "tester@test.com",
		Name:  "tester",
	}},"test","plain content goes here","<p>html content goes here</p>") 

• output source:

Content-Type: multipart/mixed; boundary=48501ac417e0fccf04f546f00de8f5728457ad1be43f2106d136b35b2ecb
MIME-Version: 1.0
Message-ID: v3BwjTuUoxQzvdppEmvYP1L10M59yKDe-7vEabzM6w8=@mailhog.example
Received: from localhost by mailhog.example (MailHog)
          id v3BwjTuUoxQzvdppEmvYP1L10M59yKDe-7vEabzM6w8=@mailhog.example; Mon, 22 Nov 2021 07:29:18 +0300
Return-Path: <sender@reearth.test>
Subject: test
To: tester@test.com

 --e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c739922c518f3
Content-Type: multipart/related; boundary=RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c739922c518f3

--RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c739922c518f3
Content-Type: multipart/alternative; boundary=ALTERNATIVE-RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c739922c518f3

--ALTERNATIVE-RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c7
Content-Type: text/plain

plain content goes here\r\n\r\n
--ALTERNATIVE-RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c7
Content-Type: text/html

<p>html content goes here</p>\r\n
--ALTERNATIVE-RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c7--

--RELATED-e2c0c686e7aa10e44c6e754ca61d286b4193e094d3ca041c739922c518f3--

Which point I want you to review particularly

Memo

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (feat/authentication-system@482518f). Click here to learn what that means.
The diff coverage is n/a.

@@                      Coverage Diff                      @@
##             feat/authentication-system      #63   +/-   ##
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  Coverage                              ?   16.50%           
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  Files                                 ?      249           
  Lines                                 ?    43270           
  Branches                              ?        0           
=============================================================
  Hits                                  ?     7141           
  Misses                                ?    35114           
  Partials                              ?     1015           

[rot1024]

Are mailers tested? See also: https://github.com/stanislas-m/mocksmtp

[rot1024]

Still draft?

[rot1024]

Codes seem ok, and do test whether smtp works well with https://github.com/mailhog/MailHog