Tokens can be cached beyond the lifetime of the (http) transport. (#834)

src/ModelContextProtocol.Core/Authentication/ITokenCache.cs

@@ -0,0 +1,17 @@
+namespace ModelContextProtocol.Authentication;
+
+/// <summary>
+/// Allows the client to cache access tokens beyond the lifetime of the transport.
+/// </summary>
+public interface ITokenCache
+{
+    /// <summary>
+    /// Cache the token. After a new access token is acquired, this method is invoked to store it.
+    /// </summary>
+    ValueTask StoreTokensAsync(TokenContainer tokens, CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Get the cached token. This method is invoked for every request.
+    /// </summary>
+    ValueTask<TokenContainer?> GetTokensAsync(CancellationToken cancellationToken);
+}