feat: improve API for sending html and plain emails

[whitehat101]

I'd like to send multipart emails with text and html using Jinja templates. This isn't well documented, but I found this PR #179 that enables basic features:

It ends up looking something like this:

message = MessageSchema(
    subject="Fastapi-Mail module",
    recipients=["[email protected]"],
    template_body={"name": "John Smith"},
    subtype=MessageType.html,
    alternative_body="This is a test email, but not from a template.",
    multipart_subtype=MultipartSubtypeEnum.alternative,
)
await Mailer.send_message(message, template_name="email.html.jinja")

I'd like to use Jinja templates for both. This PR allows it with a cleaner syntax.

message = MessageSchema(
    subject="Fastapi-Mail module",
    recipients=["[email protected]"],
    template_body={"name": "John Smith"},
    subtype=MessageType.html
)
await Mailer.send_message(message, html_template="email.html.jinja", plain_template="email.txt.jinja")

Where the template_body is passed into both versions of the template. It would be nice to have subtype become optional, but I don't want to make potentially breaking changes to the API.