chore: add hrynco common library solution

- add the standalone HrynCo.Common solution and projects
- include the shared common library source and tests
- add package metadata and a repo gitignore

HrynCo.Common/HealthChecks/Defaults/DbOptions.cs

@@ -0,0 +1,9 @@
+namespace HrynCo.Common.HealthChecks.Defaults;
+
+using System.ComponentModel.DataAnnotations;
+
+public class DbOptions
+{
+    [Required]
+    public string? ConnectionString { get; set; } = null!;
+}