Add FunctionalInterface annotation where appropriate

[fmbenhassine]

Several interfaces (like ItemReader, ItemProcessor, etc) do qualify as functional interfaces. This issue is to add the java.lang.FunctionalInterface annotation where appropriate.

[amit-github-personal]

Hello @benas, can I submit a PR for this?

[fmbenhassine]

Hi @amit-github-personal , thank you for your offer to help! There is already a PR for this issue: #4108 . If you want to help, you can for example review it and share your feedback if any. Thanks again.