ops.files.template: allow custom jinja2 template loaders

[DaRasch]

Implements the feature requested in #1485

It is now possible to override the default template loader used in the files.template operation by passing a custom instance as shown in the example in the issue.

I did not add any new tests, as I did not understand how the test system works. I also did not find any tests for the function I changed 🤷

• Pull request is based on the default branch (3.x at this time)
• Pull request includes tests for any new/updated operations/facts
• Pull request includes documentation for any new/updated operations/facts
• Tests pass (see scripts/dev-test.sh)
• Type checking & code style passes (see scripts/dev-lint.sh)

[Fizzadar]

Very nice, thank you @DaRasch!

Testing this function via the usual JSON/YAML tests is non-trivial, so OK to skip for now.