Global E-Textbook Rental Market Size, Share & Demand Report By End-User (K–12, University Students, Corporate Learners, Self-Learners), By Revenue Model (Subscription, Pay-As-You-Go, Institutional Licensing), By Product Type (STEM, Business & Economics, Humanities, Law, Vocational), By Delivery Mode (Mobile, Tablet, Desktop, Cloud-Based Access), By Region & Segment Forecasts, 2025–2030