[R2] Add aws4fetch example

Closes #5095

Holding off on presigned examples due to a bug in aws4fetch - refer to mhart/aws4fetch#35

content/r2/examples/aws4fetch.md

@@ -0,0 +1,70 @@
+---
+title: Configure `aws4fetch` for R2
+summary: Example of how to configure `aws4fetch` to use R2.
+pcx-content-type: configuration
+weight: 1001
+layout: example
+---
+
+{{<render file="_keys.md">}}
+
+JavaScript or TypeScript users may continue to use the [`aws4fetch`](https://www.npmjs.com/package/aws4fetch) npm package as per normal.
+This package uses the `fetch` and `SubtleCrypto` APIs which you will be familiar with when working in environments like browsers or
+Cloudflare Workers.
+
+You must pass in the R2 configuration credentials when instantiating your `S3` service client:
+
+```ts
+import { AwsClient } from "aws4fetch";
+
+const R2_URL = `https://${ACCOUNT_ID}.r2.cloudflarestorage.com`;
+
+const client = new AwsClient({
+  accessKeyId: ACCESS_KEY_ID,
+  secretAccessKey: SECRET_ACCESS_KEY,
+  service: "s3",
+  region: "auto",
+});
+
+const ListBucketsResult = await client.fetch(R2_URL);
+console.log(await ListBucketsResult.text());
+// <ListAllMyBucketsResult>
+//     <Buckets>
+//         <Bucket>
+//             <CreationDate>2022-04-13T21:23:47.102Z</CreationDate>
+//             <Name>user-uploads</Name>
+//         </Bucket>
+//         <Bucket>
+//             <CreationDate>2022-05-07T02:46:49.218Z</CreationDate>
+//             <Name>my-bucket-name</Name>
+//         </Bucket>
+//     </Buckets>
+//     <Owner>
+//         <DisplayName>...</DisplayName>
+//         <ID>...</ID>
+//     </Owner>
+// </ListAllMyBucketsResult>
+
+const ListObjectsV2Result = await client.fetch(`${R2_URL}/my-bucket-name?list-type=2`)
+console.log(await ListObjectsV2Result.text())
+// <ListBucketResult>
+//   <Name>my-bucket-name</Name>
+//   <Contents>
+//     <Key>cat.png</Key>
+//     <Size>751832</Size>
+//     <LastModified>2022-05-07T02:50:45.616Z</LastModified>
+//     <ETag>"c4da329b38467509049e615c11b0c48a"</ETag>
+//     <StorageClass>STANDARD</StorageClass>
+//   </Contents>
+//   <Contents>
+//     <Key>todos.txt</Key>
+//     <Size>278</Size>
+//     <LastModified> 2022-05-07T21:37:17.150Z</LastModified>
+//     <ETag>"29d911f495d1ba7cb3a4d7d15e63236a"</ETag>
+//     <StorageClass>STANDARD</StorageClass>
+//   </Contents>
+//   <IsTruncated>false</IsTruncated>
+//   <MaxKeys>1000</MaxKeys>
+//   <KeyCount>2</KeyCount>
+// </ListBucketResult>
+```