Motivation:

Armeria does not automatically delete the uploaded files, so users
should manually remove the temporary files themselves.
It would be useful if we provided some options for how to delete
multipart temporary files.

Related: line#5652

Modifications:

- Add `MultipartRemovalStrategy` that is used to determine how to delete
  multipart files. For now, three options are supported.
  - NEVER
  - ON_RESPONSE_COMPLETION
  - ON_JVM_SHUTDOWN
- Add builder methods to server/virtualhost/service builders.

Breaking changes:

- Multipart temporary files are now automatically removed when a
response is fully sent. If you want to keep the existing behavior,
use `MultipartRemovalStrategy.NEVER.

Result:

You can now specify when to remove multipart temporary files using
`MultipartRemovalStrategy`.

```java
Server
  .builder()
  .multipartRemovalStrategy(MultipartRemovalStrategy.ON_RESPONSE_COMPLETION)
```