Allow setting the local P2 cache dir in the Spotless Gradle plugin

[bric3]

This adds a way to control in which directory the Eclipse based formatters will be cached. This PR adds a cacheDirectory method in the config of each Eclipse based steps.

The reason is that, by default the eclipse resolver will place the downloaded artifacts in ~/.m2/repository/dev/equo/p2-data, this folder is a bit alien in Gradle based build. And having it nested in the maven repository structure makes it a bit harder to manage in a CI.

Given it's a Gradle related change, I choose to not add this under the maven plugin since .m2/ is already maven managed.

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[jochenberger]

Maybe the Gradle plug-in could also set a more sensible default?

[jochenberger]

Somewhere inside $GRADLE_USER_HOME for example?

[bric3]

I hesitated to do that, as it's a breaking change.

[jochenberger]

IMHO, if we just break a potentially existing cache, it's not so much of an issue. But maybe you're right. 🤷🏼‍♂️