U.S. Physician Groups Market Size, Share & COVID-19 Impact Analysis, By Number of Physicians (2 to 10, 11 to 24, 25 to 50, and More than 50), By Ownership (Physician-owned, Hospital-owned, and Others), By Specialty (Primary Care, Pediatrics, OB/GYN, Ophthalmology, Orthopedics, Psychiatry, Cardiology, Dermatology, and Others), By Payor (Public Health Insurance, and Private Health Insurance/Out-of-pocket), By Type (Single-specialty, and Multi-specialty), and Country Forecasts, 2023-2030