Add option to cleanup multipart temp files

[qavid]

In non-streaming mode, upload of file larger than DefaultPartHttpMessageReader#maxInMemorySize results in storing uploaded file to a temporrary file. These temporary files are never deleted, if there is no system job/script which is cleaning this temp folder.

We are running our applications on docker images, where the only running process is java and therefore we are running out of space.

Please consider adding option to cleanup (directly or indirectly) uploaded multipart files. Similar option is available in Spring MVC (MultipartResolver#cleanupMultipart).

[poutsma]

Introducing an option to automatically clean up these files is tricky, and might need several iterations to get right.

Because the 5.3.x branch needs to be stable, we will resolve this issue in two stages:

• In 5.3.13, we will introduce a Part::delete method that allows you to explicitly remove the files belonging to that part.
• In 6.0 (see Automatically clean up multipart temp files #27633), we will introduce a way to atomically remove these temporary files after the request is handled.

[poutsma]

This issue was fixed in 694db22, but the commit message referred to an unrelated issue.