Fixes #3859

Problem

We want to keep dependencies to an absolute minimum as this is a low level library. However we make an exception for a few selected Guava classes which are shaded in.

There's a potential risk where someone uses a new Guava class in a pull request, and either:

1. They are unaware that they should, where possible, use a non-Guava version of the method (which is different to their usual developer workflow)
2. Or they really must use a Guava class, we forget to shade in the new class in the Gradle build file, causing functionality to break

Solution

This PR adds an Archunit test to detect new Guava class usage and produce a helpful error message to guide contributors to 1. use non-Guava versions where possible or 2. adjust the Gradle build file to shade in the new class

Sample output

Example of deliberately failing pipeline (before I added the full set of exemptions): https://github.com/graphql-java/graphql-java/pull/3869/checks?check_run_id=39620708772

GuavaLimitCheck > should identify which classes use prohibited Guava dependencies STANDARD_OUT
    Prohibited Guava class usage found:

    Class: graphql.execution.ExecutionStrategy uses these prohibited Guava classes:
      - JavaClass{name='com.google.common.collect.Maps'}

    Class: graphql.schema.diffing.DiffImpl uses these prohibited Guava classes:
      - JavaClass{name='com.google.common.collect.Multiset'}
      - JavaClass{name='com.google.common.collect.HashMultiset'}
      - JavaClass{name='com.google.common.collect.Multisets'}

    Either:
    1. Preferred option: Replace them with alternatives that don't depend on Guava
    2. If absolutely necessary: Add these Guava classes to the shade configuration in the build.gradle file

Note

We don't yet use anything from the math or primitives packages directly, but these are transitively used by collect, so they also need to be shaded in

 relocate('com.google.common', 'graphql.com.google.common') {
        include 'com.google.common.collect.*'
        include 'com.google.common.base.*'
        include 'com.google.common.math.*'
        include 'com.google.common.primitives.*'
    }