Generalise description field

[siljamardla]

The current spec specifically suggests two types of descriptions.

    # Optional: Human-readable description of the semantic model
    description: string

    # Optional: Additional context for AI tools (e.g., custom prompts, instructions)
    ai_context: string

My experience shows that I would like to have the description split in at least two parts:

1. A short business-oriented description, to be used in user-facing tools. For example an info button in a BI tool, something that pops up when you hover on a field etc.
2. A (sometimes much longer) technical or deep description that provides all the detailed context on the field/measure, known issues and caveats etc. I do want this field available in my governance tools, to be used by the power users. I do not want this information exposed in user-facing tools.

I see there's a specific emphasis on AI in the OSI spec. I would simply generalise to descriptions having different types, having 3-5 types of descriptions specified in OSI specs, but enabling other types as well. For example:

    description: 
      short_description: string 
      detailed_description: string
      ai_context: string
      my_super_custom_description: string