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/Security/ISecretProtector.cs

@@ -0,0 +1,8 @@
+namespace HrynCo.Common.Security;
+
+public interface ISecretProtector
+{
+    string Protect(string plaintext);
+
+    string Unprotect(string protectedValue);
+}