Add SMTP.com email provider

[pcbeingused333]

Adds SMTP.com as a transactional email provider.

Posts to https://api.smtp.com/v4/messages. Their payload differs from the other providers: recipients live under recipients, senders under originator, addresses use an address key instead of email, and the content is an array of MIME parts under body.parts.

SMTP.com requires a sender channel on every request, so it is exposed as a provider option (config.providers.smtpcom.channel) and raises a clear Courrier::ArgumentError when missing, following the Loops pattern.

Base64 is done with Array#pack("m0") rather than Base64.strict_encode64, to avoid adding the base64 bundled gem as a dependency on Ruby 3.4+.

• lib/courrier/email/providers/smtpcom.rb — the provider
• lib/courrier/email/provider.rb — require + registry entry
• README.md — provider list plus the channel configuration example
• test/courrier/email/providers/smtpcom_test.rb — body formatting, auth header, registry lookup, dropped optional fields, and the missing-channel error

Full suite passes locally: 105 runs, 200 assertions, 0 failures, 0 errors, 0 skips.

Part of #35 — ticks the SMTP.com box.

Three things worth flagging for review:

1. Authentication. smtp.com's docs blocked my requests, so the Authorization: Basic base64("api:<key>") scheme comes from their official PHP SDK (SmtpSdk\HttpClient\HttpRequest), not from the docs. Worth confirming against a live account.
2. Part encoding. Their docs say body parts default to base64, but their own SDK sends raw content with no encoding set. I went explicit — "encoding" => "base64" with encoded content — as the unambiguous reading.
3. Naming. I used smtpcom / Smtpcom for consistency with the existing Smtp2go. Happy to rename if you'd prefer something else.