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/Caching/ISessionPromptStore.cs

@@ -0,0 +1,19 @@
+namespace HrynCo.Common.Caching;
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+public interface ISessionPromptStore
+{
+    Task<string?> GetAsync(Guid userId, string provider, CancellationToken cancellationToken = default);
+
+    Task RemoveAsync(Guid userId, string provider, CancellationToken cancellationToken = default);
+
+    Task SaveAsync(
+        Guid userId,
+        string provider,
+        string prompt,
+        TimeSpan ttl,
+        CancellationToken cancellationToken = default);
+}