KAFKA-19964: API Versions Response returns minimum metadata version for finalized level when no quorum exists

[mannoopj]

FeaturesPublisher's default finalizedFeatures assumes a metadata version of 7 when there is no finalized level for metadata.version. Instead of defaulting to 7, it should not report a value or report -1 indicating it is unknown.

See https://lists.apache.org/thread/ytmo7b9vmy46n6p47l1sxx6ftvdhht5b for the context as part of the KIP-1170 discussion.

Implemented by changing finalizedFeatures to Optional in FeaturesPublisher