CellIterator: add "lastContributionTimestamp" to OSMEntitySnapshot & decouple from Grid implementation

[tyrasd]

Description

Includes the following aspects of the CellIterator class:

• return the lastContributionTimestamp for each entity snapshot (i.e. the timestamp when the OSM entity was last touched before the snapshot timestamp)
• decouple CellIterator from grid implementation: It now accepts an OSHEntitySource (i.e. a stream of OSHEntity object plus the corresponding bounding box) instead of a raw grid cell object. This makes it easier to swap this out in the future with alternative backends that don't rely on this exact implementation of the grid
• refactor internal "data holding" classes to records

Checklist

• My code follows the code-style rules, and I have checked on the static analyses and benchmark (if applicable) results
• I have commented my code
• I have written javadoc (required for public classes and methods)
• I have added sufficient unit tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md
• I have adjusted the examples or created an issue in the corresponding repository
• I have adjusted the benchmark or created an issue in the corresponding repository

[rtroilo]

thank you 🤗