WIP: Postmark integration

[peterpacket]

Got some code for us? Awesome 🎊!

Please include a description of your change & check your PR against this list, thanks!

• There's a clear use-case for this code change, explained below
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test:all and yarn lint)

We appreciate your contribution!

[Copilot]

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Comments suppressed due to low confidence (4)

ghost/postmark-client/lib/PostmarkClient.js:8

• Ensure that PostmarkClient implements all the required functions defined in MailAdapterBase (such as 'send', 'fetchEvents', etc.) to fully satisfy the adapter interface.

module.exports = class PostmarkClient extends MailAdapterBase {

ghost/core/core/server/services/lib/MailgunClient.js:8

• Ensure that MailgunClient implements all the required functions declared by MailAdapterBase in order to comply with the adapter contract.

module.exports = class MailgunClient extends MailAdapterBase {

ghost/core/core/server/services/adapter-manager/index.js:19

• [nitpick] Verify that registering the abstract MailAdapterBase as the mail adapter does not cause issues when resolving concrete implementations; consider whether concrete adapters should be registered instead.

adapterManager.registerAdapter('mail', require('../../adapters/mail/MailAdapterBase'));

ghost/core/core/server/services/email-service/EmailServiceWrapper.js:5

• [nitpick] Confirm that the updated adapterManager.getAdapter call correctly handles all mail provider cases, especially for 'postmark', which was previously explicitly instantiated.

const adapterManager = require('../adapter-manager');