fetchpost: add --globals-json option

[jqnatividad]

   -j, --globals-json <file>   A JSON file containing global variables.
                               When posting as an HTML Form, this file is added to the Form data.
                               When constructing a payload using a MiniJinja template, the JSON
                               properties can be accessed in templates using the "globals" namespace
                               (e.g. {{globals.api_key}}, {{globals.base_url}}).