feat: Implement mailgun provider

[SivWatt]

👏 解決掉的 issue / Resolved Issues

• close #197

📝 相關的 issue / Related Issues

• 191

⛏ 變更內容 / Details of Changes

• mail.go - 新增 case 呼叫新增 mailgun provider
• mail_test.go - 新增兩個測試關於 NewMailProvider()
• mailgun.go - mailgun provider 實作
• mailgun_test.go - 測試 mailgun provider

其他

關於 mailgun provider test，我是用自己起一個 https server 的作法去測，這樣 Send 成功以及失敗就都可以被測到。 測試成功的部份有在 handler function 裡面對 request 做一些檢查，如果覺得這個部分不需要，其實也可以拿掉。 另外我新增了 mail_test.go 的想法是覺得 NewMailProvider() 裡面的邏輯 應該是要在這個 test file 裡面驗證。

[codecov-commenter]

Codecov Report

Merging #207 (6c2c208) into development (5111844) will increase coverage by 0.87%. The diff coverage is 81.81%.

@@               Coverage Diff               @@
##           development     #207      +/-   ##
===============================================
+ Coverage        45.30%   46.18%   +0.87%     
===============================================
  Files               27       28       +1     
  Lines             1342     1375      +33     
===============================================
+ Hits               608      635      +27     
- Misses             646      649       +3     
- Partials            88       91       +3     

Impacted Files	Coverage Δ
internal/mail/mailgun.go	80.64% <80.64%> (ø)
internal/mail/mail.go	58.33% <100.00%> (+8.33%)	:arrow_up:

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[y2468101216]

LGTM @PichuChen