Enable annotation engine as plugin

[bric3]

Currently AnnotationEngine can be overrriden with the deprecated MockitoConfiguration, in order to be more consistent with the new way, I propose to get the AnnotationEngine from the PluginRegistry.

Regarding backward compatible behavior, if the MockitoConfiguration class exists and can be seen then Mockito will select the engine of this configuration instead of the Plugins one. If MockitoConfiguration class don't exist then Mockito wil chose the Plugins one.

Bonus I extended the classloader util to support some tests.

[codecov-io]

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[bric3]

For reference last commit skips static final fields otherwise the dead simple field copier raises an IllegalAccessException

java.lang.IllegalAccessException: Can not set static final [Z field org.mockitoutil.ClassLoadersTest$1.$jacocoData to [Z
	at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:73)
	at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:77)
	at sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl.set(UnsafeQualifiedStaticObjectFieldAccessorImpl.java:77)
	at java.lang.reflect.Field.set(Field.java:741)
	at org.mockitoutil.ClassLoaders$ClassLoaderExecutor.reloadTaskInClassLoader(ClassLoaders.java:153)
	... 6 more

This part of the classloader test have some limitations, but it should be ok for simple tests.

[bric3]

I suggest to review this PR commit by commit.

[raphw]

I think this is the way to go. Much better to configure this with the plugin mechanism. Implementations looks good!