feat: add message mapping from MIME to Graph JSON format

[sgabb]

Understanding the possible issues with a custom mapping, the problem with previous implementation was that Django templates for html emails could not be used.
By adding a specific setting, we can maintain the plain mode for MIME email sending, and also enable the use of more advanced templates and the exploit of the Graph API.

I have added a few utility methods in MSGraphBackend to separate mapping and setting of headers and data for requests.

[danielniccoli]

Ciao Gabrio,

thanks for the PR. This looks promising and like something I want to merge, but I will need some days until I get time to review it.

Cheers,
Daniel

[sgabb]

That's perfect, no hurry at all.

[danielniccoli]

Ciao Gabrio,

sorry for the long wait on this! So HTML actually already works with the MIME path — EmailMultiAlternatives + attach_alternative(html, "text/html") and Graph renders it fine. Got a repro where it didn't for you? Curious if it's a real bug.

One thing on the JSON side: for normal mail MIME does everything JSON does and more, so that path ends up dropping stuff (text fallback, reply_to, extra_headers, inline images). JSON only really buys you Graph-specific things like reference attachments or saveToSentItems, which this doesn't touch.

Let me know if there's a case this package does not cover that you would like to add.

Saluti!

Daniel