MINOR: Support java.util.Optional in the auto-generated protocol

[dajac]

The AK protocol often relies on sentinel value (e.g. null or -1) to indicate that a field was not provided. Usually, these sentinels are turned into java.util.Optional that is empty when the field is equal to the sentinel. Until now, we were doing this transformation while mapping the Struct to internal Request or Response classes. With the generated protocol, we would like to get rid of these internal classes and also avoid spending time to construct them.

This PR is an attempt to support java.util.Optional in the auto-generated classes. A field can be marked as optional that indicates to the generator that its type must be wrapped in an Optional.

When an optional is written, its internal value is used or its default if the optional is undefined. When an option is read, the value read is used or it defaults to an empty optional if the default value is read.

I did a deep integration by changing all the places where values are written or read to do the above conversions. I just wrote it as a proof of concept to discuss this further. The code may be simplified.

An alternative to this would be to only provide getters and setters which convert the internal value to an Optional and vice versa. That would limit the scope of the change. The downside would be that every time the getters or the setters are called, it requires to do the conversion.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[dajac]

PR is outdated. Closing it.