Consider a single module for Flow and Hilla starters

[heruan]

With #247 the Flow and Hilla starters now share the same Maven com.vaadin group and contain overlapping classes.

Initially the idea of having Flow and Hilla starters as a single module was discarded to enforce product identity separation between Vaadin and Hilla and have the Hilla modules within the dev.hilla group.

Now that very premise has been overturned and we might consider again merging the two starters into a single module, e.g. using optional dependencies of Flow and Hilla and load the related classes on need.

This would simplify the adoption of the kit with a single com.vaadin:observability-kit-starter module.