Add a mechanism for easily loading third party template tag libraries

[lunkwill42]

Is your feature request related to a problem? Please describe.

We constantly find ourselves writing new template tags to make our template logic more concise. Some of these functions are accepted upstream, while some of them are very specific to our own setup and do not belong upstream.

Currently, we need to patch CNaaS-NMS locally to add our own template tags, which becomes rather tedious every time we need to do it, or every time we need to upgrade CNaaS-NMS.

Describe the solution you'd like

It would be much better if there was a mechanism or option to configure a list of third party Python modules with template tag functions, to import at runtime and make available when rendering Jinja2 templates.

Describe alternatives you've considered

Keep using the tedious manual patch management.

[lunkwill42]

IIRC, at some point I discussed with @indy-independence the option of just manipulating the Python environment to preload our own modules. Importing a module with tags made especially for CNaaS-NMS is enough, as these modules use CNaaS-NMS decorators to register the functions as tags to be made available to Jinja2 templates.