Support sending Calendar event with API

[morki]

In our application, we need to send calendar events together with our emails. For now we are using Simple Java Mail to compose our messages and send with SMTP.

We would like to use Mox Web API to send those directly, but in API documentation there is no field for calendar part.

Here is nice structure compositor to see how to integrate calendar part into multipart message: https://www.simplejavamail.org/rfc-compliant.html#section-explore-multipart

I would like to propose addition to API:

type Message struct {
    //...
    ICalendar string
}

or maybe more generaly some AlternativeParts array with content type and body.