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>

docs/modules/ROOT/pages/erc1155.adoc

@@ -112,7 +112,9 @@ The JSON document for token ID 2 might look something like:
 
 For more information about the metadata JSON Schema, check out the https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1155.md#erc-1155-metadata-uri-json-schema[ERC-1155 Metadata URI JSON Schema].
 
-NOTE: you'll notice that the item's information is included in the metadata, but that information isn't on-chain! So a game developer could change the underlying metadata, changing the rules of the game!
+NOTE: You'll notice that the item's information is included in the metadata, but that information isn't on-chain! So a game developer could change the underlying metadata, changing the rules of the game! 
+
+TIP: If you'd like to put all item information on-chain, you can extend ERC721 to do so (though it will be rather costly) by providing a xref:utilities.adoc#base64[`Base64`] Data URI with the JSON schema encoded. You could also leverage IPFS to store the URI information, but these techniques are out of the scope of this overview guide
 
 [[sending-to-contracts]]
 == Sending Tokens to Contracts

docs/modules/ROOT/pages/erc721.adoc

@@ -79,7 +79,9 @@ This `tokenURI` should resolve to a JSON document that might look something like
 
 For more information about the `tokenURI` metadata JSON Schema, check out the https://eips.ethereum.org/EIPS/eip-721[ERC721 specification].
 
-NOTE: you'll notice that the item's information is included in the metadata, but that information isn't on-chain! So a game developer could change the underlying metadata, changing the rules of the game! If you'd like to put all item information on-chain, you can extend ERC721 to do so (though it will be rather costly). You could also leverage IPFS to store the tokenURI information, but these techniques are out of the scope of this overview guide.
+NOTE: You'll notice that the item's information is included in the metadata, but that information isn't on-chain! So a game developer could change the underlying metadata, changing the rules of the game! 
+
+TIP: If you'd like to put all item information on-chain, you can extend ERC721 to do so (though it will be rather costly) by providing a xref:utilities.adoc#base64[`Base64`] Data URI with the JSON schema encoded. You could also leverage IPFS to store the tokenURI information, but these techniques are out of the scope of this overview guide.
 
 [[Presets]]
 == Preset ERC721 contract

docs/modules/ROOT/pages/utilities.adoc

@@ -99,6 +99,53 @@ Want to check if an address is a contract? Use xref:api:utils.adoc#Address[`Addr
 
 Want to keep track of some numbers that increment by 1 every time you want another one? Check out xref:api:utils.adoc#Counters[`Counters`]. This is useful for lots of things, like creating incremental identifiers, as shown on the xref:erc721.adoc[ERC721 guide].
 
+=== Base64
+
+xref:api:utils.adoc#Base64[`Base64`] util allows you to transform `bytes32` data into its Base64 `string` representation. 
+
+This is specially useful to build URL-safe tokenURIs for both xref:api:token/ERC721.adoc#IERC721Metadata-tokenURI-uint256-[`ERC721`] or xref:api:token/ERC1155.adoc#IERC1155MetadataURI-uri-uint256-[`ERC1155`]. This library provides a clever way to serve URL-safe https://developer.mozilla.org/docs/Web/HTTP/Basics_of_HTTP/Data_URIs/[Data URI] compliant strings to serve on-chain data structures.
+
+Consider this is an example to send JSON Metadata through a Base64 Data URI using an ERC721:
+
+[source, solidity]
+----
+// contracts/My721Token.sol
+// SPDX-License-Identifier: MIT
+
+import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
+import "@openzeppelin/contracts/utils/Strings.sol";
+import "@openzeppelin/contracts/utils/Base64.sol";
+
+contract My721Token is ERC721 {
+    using Strings for uint256;
+
+    constructor() ERC721("My721Token", "MTK") {}
+    
+    ...
+
+    function tokenURI(uint256 tokenId)
+        public
+        pure
+        override
+        returns (string memory)
+    {
+        bytes memory dataURI = abi.encodePacked(
+            '{',
+                '"name": "My721Token #', tokenId.toString(), '"',
+                // Replace with extra ERC721 Metadata properties
+            '}'
+        );
+
+        return string(
+            abi.encodePacked(
+                "data:application/json;base64,", 
+                Base64.encode(dataURI)
+            )
+        );
+    }
+}
+----
+
 === Multicall
 
 The `Multicall` abstract contract comes with a `multicall` function that bundles together multiple calls in a single external call. With it, external accounts may perform atomic operations comprising several function calls. This is not only useful for EOAs to make multiple calls in a single transaction, it's also a way to revert a previous call if a later one fails.