feat: add FastMail.get_message() to return the prepared message without sending

[tungyhardDevOps]

Closes #234

Motivation

#234 asks for a way to sign outgoing mail (e.g. S/MIME) — or, as the issue itself suggests, simply to "get the prepared message" so it can be signed and sent externally. Today the fully-built email.message.Message is only created inside the private send pipeline, so there is no supported way to obtain it without sending.

Change

Adds a public async method FastMail.get_message(...) that runs the same preparation pipeline as send_message (template rendering, sender resolution, MIME construction) but returns the prepared email.message.Message instead of delivering it. It reuses the existing internal __prepare_messages_for_sending, so behaviour stays identical to send_message.

• Same arguments as send_message (message, template_name, html_template, plain_template).
• A single MessageSchema returns one message; a list returns a list.
• Opens no SMTP connection and dispatches nothing.

This unblocks the S/MIME / DKIM signing use case without fastapi-mail taking on a cryptography dependency — callers sign/post-process the returned message and send it via their own transport.

Tests

tests/test_get_message.py (3 tests, all passing):

• single message → returns a prepared Message with correct headers/body, and asserts nothing is dispatched (record_messages outbox stays empty);
• list input → returns a list of prepared messages;
• templated message → template is rendered into the returned message.

Existing suite still green (tests/test_connection.py: 22 passed). black, isort, flake8 clean; mypy clean on fastmail.py.

Docs

Documented the new method in docs/getting-started.md.