Introduce a concept for "Display name"

[siljamardla]

To clearly identify fields/metrics, we sometimes use rather complex naming patterns. Very likely, they're also snake_case, which is not end user friendly. When we later start using the semantics, we will inevitably want a unified experience on display names. Where else would we define these unified display names, if not in our semantic layer.

Example:

      - name: total_sales
        display_name: "Total Sales Revenue"
        description: Total sales revenue across all transactions

[siljamardla]

Optionally hide this somewhere together with descriptions: #36

[cpimhoff]

Related to #31

[djwaldo]

+1 from the ThoughtSpot converter (#285). ThoughtSpot display names are user-facing strings ("Total Sales (AUD)") that are not valid SQL identifiers, so we need both a stable identifier and a display label to survive interchange. Today we read field.label as the display label, matching what the Databricks converter does (label → display_name). The spec describes label as "Label for categorization", which reads as something else. Either blessing label as the display-name home or adding an explicit display_name (as proposed here) would resolve it; right now two converters are already relying on an interpretation the spec doesn't quite state.