r.category: Add color option

[NishantBansal2003]

Enhancement: Add Color Option to Category Output

Fixes: #4071

Overview

This PR introduces a new color option for r.category, allowing users to retrieve category colors alongside labels. This enhancement is useful for visualization workflows where knowing both the category label and its associated color is important.

Changes Made

• Added a color option that allows specifying the output format:
  • none (default) – No color output.
  • rgb – Outputs colors in rgb(r, g, b) format.
  • hex – Outputs colors in #RRGGBB format.
  • triplet – Outputs colon-separated R:G:B values.
• Updated PLAIN output format to include color:

  1 trees, very green rgb(68, 1, 84)
  2 water, very deep rgb(253, 231, 37)
  

• Updated JSON output format to include color:

  [
      {
          "category": 1,
          "description": "trees, very green",
          "color": "rgb(68, 1, 84)"
      },
      {
          "category": 2,
          "description": "water, very deep",
          "color": "rgb(253, 231, 37)"
      }
  ]

Testing & Validation

• Existing feature tests were taken from the r.category doctest file.
• New feature tests for the color option have been added at the end.