Add MailKite email provider

[bucabay]

Summary

Adds MailKite as a transactional email provider, backed by MailKite's
POST https://api.mailkite.dev/v1/send JSON API.

What changed

• New Courrier::Email::Providers::Mailkite provider that maps the standard
  from / to / cc / bcc / reply_to / subject / text / html
  fields to the MailKite send API, authenticating with a Bearer API key.
• Comma-separated recipients are split into arrays (to, cc, bcc), which
  is what the MailKite API accepts (replyTo stays a single string).
• Registered the provider in the Courrier::Email::Provider registry.
• Added provider tests (body formatting, optional-recipient omission, auth
  header, registry availability) and updated the registry coverage test.
• Documented MailKite in the README provider list.

Usage

Courrier.configure do |config|
  config.email = {
    provider: "mailkite",
    api_key: "mk_live_..."
  }

  config.from = "devs@example.com"
end

OrderEmail.deliver to: "recipient@example.com"

Verification

rake (test + standard) passes: 104 runs, 192 assertions, 0 failures.

[bucabay]

Hi — friendly bump on this one: it adds a MailKite email provider (a Courrier::Email::Providers::Mailkite mapping to POST /v1/send, registered in the provider registry, with tests and a README entry). Open to any changes or a rebase if that helps review. Thanks for taking a look!

[eelcoj]

Thanks @bucabay. ❤️ Are you the owner/builder of MailKit? Be sure to add Courrier to your docs for your Ruby users! ✨