Base64::encode : const correctness / String by reference passing (#3314)

Avoid passing String by-value, which is slightly less efficient
as it involves a full copy-constructor/tempstring creation.

cores/esp32/base64.h

@@ -4,8 +4,8 @@
 class base64
 {
 public:
-    static String encode(uint8_t * data, size_t length);
-    static String encode(String text);
+    static String encode(const uint8_t * data, size_t length);
+    static String encode(const String& text);
 private:
 };