feat(libstore/s3-binary-cache-store): implement uploadPart()

Implement `uploadPart()` for uploading individual parts in S3 multipart
uploads:

- Constructs URL with `?partNumber=N&uploadId=ID` query parameters
- Uploads chunk data with `application/octet-stream` mime type
- Extracts and returns `ETag` from response

src/libstore/s3-binary-cache-store.cc

@@ -37,6 +37,15 @@ private:
     std::string createMultipartUpload(
         std::string_view key, std::string_view mimeType, std::optional<std::string_view> contentEncoding);
 
+    /**
+     * Uploads a single part of a multipart upload
+     *
+     * @see https://docs.aws.amazon.com/AmazonS3/latest/API/API_UploadPart.html#API_UploadPart_RequestSyntax
+     *
+     * @returns the [ETag](https://en.wikipedia.org/wiki/HTTP_ETag)
+     */
+    std::string uploadPart(std::string_view key, std::string_view uploadId, uint64_t partNumber, std::string data);
+
     /**
      * Abort a multipart upload
      *
@@ -88,6 +97,28 @@ std::string S3BinaryCacheStore::createMultipartUpload(
     throw Error("S3 CreateMultipartUpload response missing <UploadId>");
 }
 
+std::string
+S3BinaryCacheStore::uploadPart(std::string_view key, std::string_view uploadId, uint64_t partNumber, std::string data)
+{
+    auto req = makeRequest(key);
+    req.setupForS3();
+
+    auto url = req.uri.parsed();
+    url.query["partNumber"] = std::to_string(partNumber);
+    url.query["uploadId"] = uploadId;
+    req.uri = VerbatimURL(url);
+    req.data = std::move(data);
+    req.mimeType = "application/octet-stream";
+
+    auto result = getFileTransfer()->enqueueFileTransfer(req).get();
+
+    if (result.etag.empty()) {
+        throw Error("S3 UploadPart response missing ETag for part %d", partNumber);
+    }
+
+    return std::move(result.etag);
+}
+
 void S3BinaryCacheStore::abortMultipartUpload(std::string_view key, std::string_view uploadId)
 {
     auto req = makeRequest(key);