Move some entities to CacheEntry to be saved and cached to filesystem and use it's API for PluginMetada

[jonesbusy]

It's a quiete large PR sorry but was difficult to split.

This basically add the CacheEntry object into the game. It's used by the CacheManager

Testing done

Rely on automated tests (needed to adapt some of them due to changes of public API)

Tested the metadata is moved from a virtual target/plugin-metadata into the tool cache (~/.cache/). This was necessary because target folder are empty when maven clean run or on git clean operation.

This PR will also allow some more caching like the Plugin entity if we need to

This PR also add some JSON utility instead of create GSON object everywhere. I'm also not 100% happy with GSON. Perhaps we should move to Jackson this utility class that give more flexibility using annotation

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