DE-1387 Release v5

[vtopc]

Changes:

• Send(ctx context.Context, m SendableMessage) (mes string, id string, err error) now accepts interface - https://bryanftan.medium.com/accept-interfaces-return-structs-in-go-d4cab29a301b
• Mailgun client is now domain agnostic:
  • No need to pass the domain into the func mailgun.NewMailgun
  • Many methods now require passing a domain, which indicates which methods are domain-specific and which are global for your account.
    • Pay attention to that func NewMessage(domain, from, subject, text string, to ...string) *PlainMessage has a variadic to argument, so there would be no compile error if you miss passing domain.
• SetAPIBase() now returns an error.
• For the EU: do mg.SetAPIBase(mailgun.APIBaseEU) instead of mg.SetAPIBase("https://api.eu.mailgun.net/v...").