Add support for arbitrary calendar headers

[nickgartmann]

In order to get a calendar invite sent via email to show RSVP buttons, the ics file must contain a METHOD header which matches the method provided in the Content-Type header of its file attachment (See the first paragraph of the iMIP RFC: https://datatracker.ietf.org/doc/html/rfc6047#section-2.4)

This can be achieved by inserting the headers manually after the ics has been generated, but this PR adds support for arbitrary headers to be added below the PRODID header.

[nickgartmann]

After some research of other libs, it looks like they call them properties but also there is a finite list of properties that are supported (per: https://datatracker.ietf.org/doc/html/rfc5545#section-3.7). They are:

• CALSCALE
• METHOD
• PRODID
• VERSION

You have everything but METHOD already in there, so maybe instead of adding headers I should just add method as an optional param in to_ics/1? I could also add the option to set CALSCALE and VERSION manually

[lpil]

That could work, though is it clear? What does the Ruby library do there?

[nickgartmann]

They define the settable properties manually and then loop through all of the calendar properties when serializing

They call the property ip_method - they had it as method but changed it in 2014 - I suspect this change was driven to minimize confusion with ruby meta programming naming conventions. Ruby prefers the name method over functions since you can't have a standalone function not attached to an object. The reasoning here is a guess though because the commit messages don't talk about the change at all and there was not a PR that drove the change.