r.volume: Add JSON support

[NishantBansal2003]

Fixes: #5949

This PR adds JSON support to the r.volume module. The JSON output looks like:

[
            {
                "category": 217,
                "average": 118.92688424466569,
                "sum": 86288827.98632812,
                "cells": 725562,
                "volume": 8628882798.632812,
                "easting": 635325,
                "northing": 221535,
            },
            {
                "category": 262,
                "average": 108.96982328824218,
                "sum": 21650560.37020111,
                "cells": 198684,
                "volume": 2165056037.020111,
                "easting": 638935,
                "northing": 222495,
            },
]

This PR includes the following changes:

1. Adds a format option with plain, csv, and json modes for output formatting.
2. Adds format = csv option; the -f flag is now deprecated.
3. Adds a separator parameter to be able to change the unformatted report; If no separator is specified, the default is :; If the format is CSV, the separator is a comma.
4. Adds a -p flag to control whether the report is printed. Currently, the report is printed by default, but this flag will be required in the future.
5. Adds tests covering each of the new formats.
6. Adds a Pandas example to the documentation for parsing JSON output.