v.dissolve: add column2 support for multi-column dissolve

[laasyapriya-code]

Solution: Introduce optional column2 parameter that creates a composite dissolve key (column_column2) for precise feature aggregation.

Key Changes:

• Added column2 parameter with backward compatibility
• Composite column creation: column + "_" + column2
• All downstream operations updated to use composite key

-Usage:
bash
Single column (existing behavior)
v.dissolve input=roads output=roads_dissolved column=type
Multi-column (new feature)
v.dissolve input=roads output=roads_dissolved column=type column2=status

Fixes #7050

[wenzeslaus]

This will need a thorough test to be considered for merging.

[ninsbl]

Did you consider allowing multiple input in column (maybe extending the option to columns)? That would be more flexible than adding a column2 option...