smtp_batch: add feature for grouping and templating

[Lukas-Heindl]

Like discussed in #2586, here is the thing. I already did some manual testing regarding templating the subject string. For this configured

1. add extra.my_template_value to the events (I did this with the modify bot for now)
2. add that key (extra.my_template_value) to the new additional_grouping_keys parameter
3. use subject: "Prefix {{ extra_my_template_value }}"
4. activate the templating capabilites for the subject line with templating: {subject: true}

But probably some tests in the test suite will be needed for this (but I'd like to defer this one after the first feedback round)

Any comments?

Maybe issues to discuss:

• how values get hashed generically with hash_arbitrary
• passing the template_data along with each mail as a tuple
• really a warning if jinja2 couldn't be imported? (the user might not want jinja2) -- also whether to add this optional requirement to the requirements.txt?

Also I didn't get the testing environment setup correctly until now, so it would be nice if someone could trigger the pipelines yet already so I can check the outputs regarding code-style.

[Lukas-Heindl]

Feel free to have a look, but it will be some time (estimate: 1-2 weeks) until I can have a look at the failing pipelines.
My way to go would be

1. looking into what makes them fail
   • code style should pass obviously
   • failing tests here might indicate a not fully backwards compatible change
2. feedback (of course any time, also before/while 1.)
3. Write additional tests covering the new features

[e3rd]

Seems good, thanks! :)

[e3rd]

Integrate template_data into the Mail dataclass instead of passing tuples.

Add annotations in the new code for the non-trivial objects (they did not exist when I wrote it originally), ex, def build_mail(self, mail: Mail,