Allow getting module information in JSON format

Closes #243 (closed)

Required by sw/armarx/armarx-core!229 (merged), documentation!12 (merged)

Add --json to axii module show to produce JSON output without any decorations:

$ axii module show --help
 Usage: axii module show [OPTIONS] MODULE_NAME                                                                                                                                                              
                                                                                                                                                                                                            
 Show a module definition.                                                                                                                                                                                  
                                                                                                                                                                                                            
╭─ Options ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│  --json      Format the output in JSON without any decorations.                                                                                                                                          │
│  --help      Show this message and exit.                                                                                                                                                                 │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

$ axii module show armarx/ArmarXCore --json 
{
  "name": "armarx/ArmarXCore",
  "general": {
    "url": "https://git.h2t.iar.kit.edu/sw/armarx/armarx-core",
    "authors": [
      "Rainer Kartmann <rainer.kartmann@kit.edu>",
      "Christian Dreher <c.dreher@kit.edu>"
    ],
    "authors_parsed": {
      "rainer.kartmann@kit.edu": "Rainer Kartmann",
      "c.dreher@kit.edu": "Christian Dreher"
    }
  },
  "update": {
    "git": {
      "ssh_url": "git@git.h2t.iar.kit.edu:sw/armarx/armarx-core.git",
      "https_url": "https://git.h2t.iar.kit.edu/sw/armarx/armarx-core.git",
      "h2t_gitlab_slug": "sw/armarx/armarx-core",
      "submodules": false,
      "prefer_https": false
    },
    "archives": {
      "archives": []
    }
  },
  "prepare": {
    "cmake": {
      "project_name": "ArmarXCore",
      "definitions": {
        "CMAKE_C_COMPILER": "$ARMARX_C_COMPILER",
        "CMAKE_CXX_COMPILER": "$ARMARX_CXX_COMPILER"
      },
      "generator": {}
    },
    "python": {
      "packages": {
        "etc/python": {
          "relative_path": "etc/python",
          "python": "$AXII_DEFAULT_PYTHON_INTERPRETER",
          "venv_name": ".venv",
          "preinstall": [],
          "install_editable": []
        }
      }
    }
  },
  "build": {
    "cmake": {
      "env": {}
    }
  },
  "install": {
    "env": {}
  },
  "required_modules": {
    "apt/ice": {
      "features": []
    },
    "apt/jsoncpp": {
      "features": []
    },
    "apt/lmdb": {
      "features": []
    },
    "apt/sqlite3": {
      "features": []
    },
    "apt/python3-argcomplete": {
      "features": []
    },
    "# For python: (pip install zeroc-ice)": {
      "features": []
    },
    "apt/bz2": {
      "features": []
    },
    "simox": {
      "features": []
    },
    "armarx/meta/compiler": {
      "features": []
    },
    "tools/default_python_interpreter": {
      "features": []
    }
  },
  "features": {}
}

Currently, items with null value are removed before printing.