Add content length to GCP multipart complete

This commit fixes the GCP mulipart complete implementation by adding
the Content-Length header to the XML request. According to the docs,
https://cloud.google.com/storage/docs/xml-api/post-object-complete,
this header is required but wasn't being set previously.

Author: jkosh44

object_store/src/gcp/client.rs

@@ -540,6 +540,7 @@ impl GoogleCloudStorageClient {
         let response = self
             .client
             .request(Method::POST, &url)
+            .header(&CONTENT_LENGTH, data.len())
             .bearer_auth(&credential.bearer)
             .query(&[("uploadId", upload_id)])
             .body(data)