Move test plugins in a detached project so they can use the Gradle Besu plugin

[fab-10]

PR description

This PR ports the existing test plugins, used for acceptance tests, to use the Gradle plugin for Besu plugins, this way the acceptance tests are closer to the standard way of using plugins and it will allow for more plugin verification on startup to be implemented in following PR.

To achieve that the test-plugins subproject has to be moved to a new detached project, due to circular dependencies between Besu and the Gradle plugin, while trying to keep the DevUX intact as much as possible.
Basically since the Gradle plugin needs to know the version of Besu to use, and we want to use the current local version, before building anything in the detached project, Besu project must publish its artifacts to mavenLocal, for this helper tasks have been added to Besu's build.gradle to automate that part, and normally a developer should not know about them, because the normal development workflow remains the same.

Fixed Issue(s)

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• Considered documentation and added the doc-change-required label to this PR if updates are required.
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Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests
• hive tests: Engine or other RPCs modified?

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