Add support for Bulk Email API (deliver_bulk, get_bulk_status)

[ctmaloney]

Adds support for Postmark's Bulk Email API, which is now generally available.

The bulk API differs from the existing email/batch endpoint — it accepts a single message definition (subject, body, from, stream) with a Messages array of per-recipient data, rather than a full message object per recipient. This makes it significantly more efficient for large sends.

Changes

Two new methods on ApiClient:

• deliver_bulk(attributes) — sends to the email/bulk endpoint. Accepts a hash matching Postmark's bulk API shape, with a top-level Messages array for per-recipient overrides.
• get_bulk_status(bulk_request_id) — fetches the status of a submitted bulk request by ID, returning progress and submission metadata.

Both methods follow existing gem conventions: with_retries, format_response, serialize, and HashHelper.to_postmark.

Example

client.deliver_bulk(
  From: "sender@example.com",
  Subject: "Hello",
  HtmlBody: "<b>Hi there</b>",
  TextBody: "Hi there",
  MessageStream: "your-broadcast-stream",
  Messages: [
    { To: "a@example.com" },
    { To: "b@example.com" }
  ]
)