Configurable persistent index format version in the Maven plugin

[Ladicek]

The Maven plugin could expose a configuration property for setting which index version to write. It would then call IndexWriter.write(Index, int) instead of just IndexWriter.write(Index). Default would be unconfigured, which would lead to calling IndexWriter.write(Index) and hence using the latest index version.

The purpose would be to decouple Jandex version bump from Jandex persistent index format version bump. Libraries that produce and distribute a Jandex index could decide to bump Jandex without bumping the persistent index format, so that the index remains readable by older Jandex versions.

On the other hand, there's clear desire to make libraries ship latest indices, so that new Jandex features can be used, so maybe we would only allow configuring a subset of possible index versions? Say, current and current - 1, something like that.