templates: Add custom template function registration

[kroppt]

Fixes #4568

This allows plugins to add functions to the template execution engine. Such plugin modules must be placed under http.handlers.templates.functions.*, e.g. http.handlers.templates.functions.foo. The interface templates.CustomFunctions must be implemented by the plugin module in order to provide those functions.

[mholt]

This is a neat change, thanks for contributing! Let's try it out.

[mholt]

@kroppt Thank you so much for your contribution on this. By now, I needed to be able to add configuration surface for my own template extension, and AFAIK we never documented this patch in the first place and I don't know anyone else who was using this. Which is a bummer because this is actually a super-useful addition.

If it's alright with you, I've built on your work with #5939 where we add more "official" support for pluggable template components; they are now required to be specified in the config, but this is true to Caddy module conventions so that users can configure them, and enables more flexibility. Just took me a few minutes since you laid the groundwork here.

Really appreciate it!

[kroppt]

@mholt Sounds good. It makes sense that you'd have to explicitly "import" the custom functions at configure time that an extension is defining at compile time.