Allow custom ITokenProvider implementation to support header modification

Currently, the ITokenProvider interface is public, but its required method CreateHandler() is internal, which prevents consumers from creating their own implementations outside of the Zitadel.Net assembly.

Author: dennisoehme

src/Zitadel/Api/ITokenProvider.cs

@@ -34,5 +34,10 @@ static ITokenProvider ServiceAccount(
             options);
     }
 
-    internal DelegatingHandler CreateHandler();
+    /// <summary>
+    /// Creates the delegating handler for the token provider.
+    /// Made public to allow custom ITokenProvider implementations outside Zitadel.Net.
+    /// </summary>
+    public DelegatingHandler CreateHandler();
 }
+