Variable controls for index pattern in ES|QL (FROM ?index)

**What the feature is**

Variable controls for index pattern in ES|QL (FROM ?index)

**Value proposition**:

This feature targets users who create dashboards with ES|QL visualizations over data that is partitioned into multiple index patterns or streams. The main problem today is that users must create and maintain duplicate dashboards to run the same query logic against different data sources, leading to high maintenance overhead. This capability solves that by allowing a single dashboard to use a user-facing control (linked to the named parameter) to dynamically switch the entire dashboard's context to a different index pattern, significantly reducing duplication and simplifying maintenance.

**Expected outcome**:

- The ES|QL syntax will allow a named parameter to be set in the index pattern position of the FROM clause (e.g., FROM ?my_index).
- Users can link a dashboard control (like a static dropdown) to this named parameter.
- Selecting an index pattern or data stream from the control will dynamically update all linked ES|QL visualizations to query the selected data source.
- A single dashboard can now be used to visualize the same information across multiple, distinct data sources (e.g., daily indices, regional data streams).