Add extensive templating helpers based on Masterminds/sprig

[oncilla]

When writing custom templates, the current set of functions is fairly limited.

We could register the functions from https://github.com/Masterminds/sprig to offer the template authors an extensive set of helper functions. Docs of the functions is available here: https://masterminds.github.io/sprig/

If you are interested, I would be happy to send a patch.