Add Base64 library to utils (#2884)

* Add Base64 library to utils

* Fix typo on Base64 padding

* Added documentation for Base64 and references from ERC1155 and ERC721

* Updated Changelog

* Fix typo in utilities doc

* use mstore8 to improve memory accesses

* use shorter strings with encodePacked

* do not use using-for syntax, for clarity

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/mocks/Base64Mock.sol

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../utils/Base64.sol";
+
+contract Base64Mock {
+    function encode(bytes memory value) external pure returns (string memory) {
+        return Base64.encode(value);
+    }
+}