Move away from using the body var for Jinja2 templates

[floodpants]

Move away from template.render(body=foobar) to a more dynamic approach. make_dict() is there to simplify validation of various source data types (eg creating a dict from a list of tuples etc). This collectively should allow and encourage greater re-use of J2 templates throughout a given app.

BC Break: This will mean that any templates which currently use {{ body.foobar }} will need to switch to {{ foobar }}, which is a pretty noticeable BC break, however given fastapi-mail is currently v0.x I think that's likely acceptable. There are workarounds to attempt to provide both levels of functionality if a smoother transition is desired but I'm not sure they're fool proof.

[sabuhish]

Thanks one more time for bringing such a brilant changes. I think we should mention it in our docs so people can work around it, otherwise, they will have troubles. If you have time to make changes go ahead if not let me know I will make changes in docs as well. Thanks!

[floodpants]

Yep, that's a good plan. I'll make some changes to the docs over the weekend and do an updated PR

[floodpants]

Updated the docs to expand on the Jinja2 section a bit and added a note for a "legacy" method in <= 0.4.0. Let me know what you think

[sabuhish]

That's awesome! That's what I was thinking as well, thanks for the great work!