Support structured external source for variable

Currently it's possible to define a variable with a string value loaded from a text file (varname.file: filename).

For the firmware-nonfree and firmware-free source packages, we also want to know about symlinks that will point to a firmware file, so they can be mentioned in package descriptions. These are described in upstream metadata and should not be duplicated in debplate-config.yml. So there is a need to import a structured value from an external source.

We could implement loading structured data if filename ends with .json or .yml, or introduce distinct variable name extensions to specify the file format.

It might also be useful to support running a command instead of reading a file, to avoid the need for another generated file in the source package.