feat(core): define primary models and services for encryption

src/FiscalOS.Core/Security/KeyRingEntry.cs

@@ -0,0 +1,21 @@
+namespace FiscalOS.Core.Security;
+
+public sealed record KeyRingEntry
+{
+  public string KeyId { get; init; }
+  public string Key { get; init; }
+
+  private KeyRingEntry(string keyId, string key)
+  {
+    KeyId = keyId;
+    Key = key;
+  }
+
+  public static KeyRingEntry From(string keyId, string key)
+  {
+    ArgumentNullException.ThrowIfNull(keyId);
+    ArgumentNullException.ThrowIfNull(key);
+
+    return new(keyId, key);
+  }
+}