Possibility to update jinja globals with python functions

[robgui87]

I think it would be possible to have access to custom python functions in the alert_text when using alert_text_jinja as an alert type.

When we load the jinja template it could be possible to update the globals to inject a dict that points to some custom python functions.

I have had a case where a list of dictionaries is stored as a string in Elastic (maybe it's a common case, I don't know, I am kind of new to Elastalert). When I want to use jinja I haven't found a way to split it with what they offer. However it would be possible with some custom functions.

I was able to modify the code locally and it worked quite easily (code wasn't clean though). I used something like this: https://saidvandeklundert.net/2020-12-24-python-functions-in-jinja/

I am open to code the feature myself, but I am new to git/github (I have been using hg for several years though)

Thanks