Change variables in the manifest file from dictionaries to objects

[lorenzo-cavazzi]

Currently, variables are dictionaries where the key is the variable name and the values represent its description.
We should use an object instead so that we can specify other properties like type, default, and support enumerations.

Instead of summary: short summary added at the beginning of the readme file, we should have something like this:

  - name: summary
    description: short summary added at the beginning of the readme file
    type: string

For enumerations, we should have something like the following:

  - name: plugins
    description: list of plugins to install in the template
    type: enum
    enum: ["none", "VScode", "GIS", "VScode and GIS"]
    default: none

Initial support for strings and enumerations may be enough.
We could also support numbers and booleans for completeness and to have a better validation system, as well as a better UX in the UI.