[Variant] Remove explicit ObjectBuilder::finish() and ListBuilder::finish and move to `Drop` impl

[alamb]

Looking at these examples, it seems to me like the current API requires callingfinish

One problem with this is that it requires someone to remember to call finish otherwise their object will only be half written

it seems like it might be a nicer API if that just happened on drop() -- so you could use the scope of the example, like you have here, and the dropping would just complete the object

This would be a great follow on PR

Originally posted by @alamb in #7778 (comment)

[alamb]

label_issue.py automatically added labels {'parquet'} from #7843