Currency type - dynamic currency codes

[steve-hx]

Is your feature request related to a problem? Please describe.
The currency parameter on number measures/dimensions currently accepts a fixed ISO-4217 literal only:

measures:
  - name: total_amount
    sql: amount
    type: sum
    format: currency
    currency: USD        # fixed literal - cannot reference a column/dimension

This is fine when a column is always one currency. It breaks down for datasets where each row is denominated in its own currency, with the currency code stored in a companion column - a very common shape in any situation where you are dealing across currencies.

Today there is no way to tell Cube that a currency field depends on a second dimension to define the actual currency. Note: this is not about formatting the result, that should be a frontend consideration - but about being able to define at teh semantic layer a currency field when its not fixed per cube.

Describe the solution you'd like
Allow currency (or a new currency_code) to accept a field/SQL reference in addition to a literal, e.g.:

measures:
  - name: total_amount
    sql: amount
    type: sum
    format: currency
    currency:
      sql: "{CUBE}.order_currency"     # resolves per row from another column

or referencing a dimension:

    currency:
      dimension: order_currency

Behaviour asked for:

• Per-row resolution. The currency code is read from the referenced field/expression for each row, so mixed-currency result sets render correctly.
• Companion inclusion. When a currency-formatted measure is selected, Cube ensures the referenced currency-code field is available/returned (or at minimum exposes the linkage in the response annotation) so clients don't have to hardcode the pairing.
• Backward compatible. A literal currency: USD keeps working exactly as today; the field-referenced form is opt-in.
• Numeric type preserved. Values stay numeric on the wire (like Power BI's dynamic format strings, which keep the measure's data type); the currency is display metadata resolved at the edge, never a pre-formatted string baked into the value.

Describe alternatives you've considered

• meta + client glue. Stash the currency-code field name in meta and have the frontend read it, add the field to the query, and format per row. Works, but every consumer re-implements it and Cube does nothing automatically. This is our current plan as a workaround.
• Views that pair the members. A view can expose amount and order_currency together, but selecting one still doesn't pull the other, and the currency is still a fixed literal on the format — so it doesn't achieve per-row labels.
• Multi-stage switch measures (precomputed amount_usd/amount_eur/…). Selects a value for a chosen currency but the display currency attribute stays a fixed literal; doesn't give per-row labels within one result, and doesn't scale to arbitrary currencies.

Additional context

Prior art - other semantic-layer / BI tools that support this

This feature isnt supported as standard, but, does have existing implementations:

• ThoughtSpot - "Set number, date, and currency formats": https://docs.thoughtspot.com/cloud/26.4.0.cl/data-modeling-patterns · dbt tag ts_currency_type: from_column: https://docs.thoughtspot.com/cloud/26.5.0.cl/dbt-integration-metadata-tags
• Power BI - Dynamic format strings for measures: https://learn.microsoft.com/en-us/power-bi/create-reports/desktop-dynamic-format-strings · SQLBI write-up (per-cell variation): https://www.sqlbi.com/articles/introducing-dynamic-format-strings-for-dax-measures/
  There is a long standing open issue in metabase too (but not implemented AFAIK)
• Metabase - open issue perf: Debounce information schema queries to Cube Store (1.8x) #9879: Support a currency column for a Currency amount column metabase/metabase#9879 · maintainer confirmation: https://discourse.metabase.com/t/how-to-set-currency-dynamically/16410