Add entries for new PUT features to the client changelog (#1067)

* Add entries for new PUT features to the client changelog

Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>

* Remove subsection

Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>

---------

Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>

mountpoint-s3-client/CHANGELOG.md

@@ -1,8 +1,10 @@
 ## Unreleased
 
-### Other changes
-
 * Add support for copy object operation. ([#1052](https://github.com/awslabs/mountpoint-s3/pull/1052))
+* Introduce a new API (`put_object_single`) to perform single PutObject requests rather than multi-part uploads. ([#1046](https://github.com/awslabs/mountpoint-s3/pull/1046))
+* Return the new object ETag after a successful PUT request. ([#1057](https://github.com/awslabs/mountpoint-s3/pull/1057))
+* Add support for custom headers in PUT requests. ([#1059](https://github.com/awslabs/mountpoint-s3/pull/1059))
+* Add support for writing object metadata in PUT requests. ([#1062](https://github.com/awslabs/mountpoint-s3/pull/1062))
 * Address a threading issue in the s2n-tls library that could result in premature cleanup and `NULL pointer` errors. ([aws/s2n-tls#4584](https://github.com/aws/s2n-tls/pull/4584))
 * Inaccurate reporting of `s3.client.buffer_pool.primary_allocated` CRT statistic is fixed. ([awslabs/aws-c-s3#453](https://github.com/awslabs/aws-c-s3/pull/453))
 * Expose `s3.client.buffer_pool.forced_used` metric which account for buffer allocations that could exceed memory limit in the CRT buffer pool. ([#1025](https://github.com/awslabs/mountpoint-s3/pull/1025))