API: Add health check endpoint

BTCPayServer/Controllers/GreenField/HealthController.cs

@@ -0,0 +1,16 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace BTCPayServer.Controllers.GreenField
+{
+    [ApiController]
+    public class HealthController : ControllerBase
+    {
+        [AllowAnonymous]
+        [HttpGet("~/api/v1/health")]
+        public ActionResult GetHealth()
+        {
+            return Ok();
+        }
+    }
+}