v.what.rast: Add JSON support

[NishantBansal2003]

Fixes: #6024

This PR adds JSON support to the v.what.rast module. The JSON output looks like:

[
	{
		"category": 21,
		"value": 0.5
	},
	{
		"category": 22,
		"value": 1
	},
	{
		"category": 23,
		"value": 1.5
	},
	{
		"category": 24,
		"value": 2
	},
	{
		"category": 25,
		"value": 4.5
	}
]

This PR includes the following changes:

1. Adds a format option with plain, csv, and json modes for output formatting.
2. Adds separator option for CSV style output.
3. Adds tests covering each of the new formats.
4. Adds a Pandas example to the documentation for parsing JSON output.

[petrasovaa]

I would also include the json output for completeness