feat(service): add MailKite email service

[bucabay]

Description

This PR adds a new notification service for MailKite, an email API for sending and receiving transactional mail.

The service sends messages through the MailKite Send API (POST /v1/send) with a Bearer API key. It follows the shape of the Mailtrap service merged in #1045: New(apiKey, senderAddress, opts...), AddReceivers, BodyFormat(HTML|PlainText), and functional options WithSenderName, WithReplyTo, WithBaseURL, WithHTTPClient. Standard library only (net/http), so no new dependencies.

Disclosure: I build MailKite. I am happy to maintain this service going forward (API changes, issues, follow-ups) and will watch for anything filed against service/mailkite.

Motivation and Context

Users of notify who send with MailKite currently have to go through the generic SMTP mail service. This adds first-class support alongside the other email providers (Amazon SES, Mailgun, Mailtrap, SendGrid) so the API key path works directly, with a reply-to option since notification replies usually belong somewhere other than the sender address.

How Has This Been Tested?

• Table-driven unit tests in service/mailkite/mailkite_test.go using httptest.Server to assert the request method, path, Authorization: Bearer … / Content-Type headers, and the JSON body (from with and without a display name, multiple receivers, reply-to, HTML vs plain text), plus constructor/options, error-status and transport-error cases. go test -race -cover ./service/mailkite/... passes with 95.2% coverage.
• go build ./... and go vet ./service/mailkite/... clean.
• golangci-lint v2.7.2 with this repo's .golangci.yaml: 0 issues. gofumpt, golines --shorten-comments -m 120 and gofmt report no changes.
• The request body (from, to[], subject, html/text, replyTo) is the same shape MailKite's own Go SDK sends to the production API; I have not wired a live send into the test suite, so the tests are all against the local mock server.

Screenshots / Output (if appropriate):

ok  	github.com/nikoksr/notify/service/mailkite	1.435s	coverage: 95.2% of statements
0 issues.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.