Use a constant suffix for temporary GCS part files. (#5308)

[SC-54146](https://app.shortcut.com/tiledb-inc/story/54146/use-a-constant-suffix-for-the-gcs-temporary-part-files)

This PR changes the format of the names of temporary GCS part files from
`<object_path>__tiledb_<n>` to `<object_path>.part<n>.tmp`. Because the
new names have a constant `.tmp` suffix, this enables GCS users to set a
[lifecycle rule](https://cloud.google.com/storage/docs/lifecycle) to
automatically delete them after a certain time if they happen to be left
over (usually they get deleted at the end of the upload operation, but
it might not happen if the operation was interrupted).


![image](https://github.com/user-attachments/assets/318717fb-6b68-47de-a69d-0fa6e5423053)

Documentation will also be updated. For consistency we should also use
`.tmp` for any kind of future temporary objects we upload.

---
TYPE: IMPROVEMENT
DESC: Temporary files (currently used by GCS only) now end with `.tmp`,
allowing you to set a lifecycle rule to automatically delete them after
a certain time if they happen to be left over.

tiledb/sm/filesystem/gcs.h

@@ -406,7 +406,7 @@ class GCS {
     std::string next_part_path() {
       const uint64_t part_id = next_part_id_++;
       const std::string part_path =
-          object_path_ + "__tiledb_" + std::to_string(part_id);
+          object_path_ + ".part" + std::to_string(part_id) + ".tmp";
       part_paths_.emplace_back(part_path);
       return part_path;
     }