[request] support adding index on incremental models

[dberardo-com]

currently i am using

-- ON_VIRTUAL_UPDATE_BEGIN;
-- JINJA_STATEMENT_BEGIN;
-- CREATE INDEX IF NOT EXISTS idx_incremental_tdata_ts ON {{ this_model }} (sampling_date_time);
-- CREATE INDEX IF NOT EXISTS idx_incremental_tdata_device_ts ON {{ this_model }} (device_id, sampling_date_time);
-- JINJA_END;
-- ON_VIRTUAL_UPDATE_END;

on an incremental table and it gives error

_duckdb.Error: Failed to execute query "BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
CREATE INDEX idx_incremental_tdata_ts ON telemetry_data_sqlmesh.incremental_telemetry_data(sampling_date_time)": ERROR: "incremental_telemetry_data" is not a table or materialized view