Text this: Exploring the correlations of lung ultrasound with delirium and other clinical outcomes in older patients with respiratory failure admitted in acute geriatric units (ECO-AGE): protocol for a multicentre, prospective, observational study from the GRETA Group (Gruppo di Ricerca in Ecografia Toracica nell’Anziano) of the Italian Society of Gerontology and Geriatrics (SIGG)