Crypto: HMAC-SHA256 cleanup

Add byte[] key method to reduce object churn in NTCP2
Un-deprecate in context

core/java/src/net/i2p/I2PAppContext.java

@@ -746,8 +746,9 @@ public class I2PAppContext {
         }
     }
 
-    /** @deprecated used only by syndie */
-    @Deprecated
+    /**
+     * Un-deprecated in 0.9.38
+     */
     public HMAC256Generator hmac256() {
         if (!_hmac256Initialized)
             initializeHMAC256();