Support non-public BeanOverrideProcessor implementations in the TestContext framework

[sbrannen]

Overview

A BeanOverrideProcessor will typically only be referenced via @BeanOverride(MyCustomProcessor.class) which is used as a meta-annotation on a user's/project's custom composed annotation (analogous to @TestBean and @MockitoBean).

In light of that, there's seldom a reason to make such a type public.

In order to allow users to make BeanOverrideProcessor implementations package-private, we should ensure that BeanOverrideParser is able to instantiate non-public BeanOverrideParser types.

We should also investigate whether TestBeanOverrideProcessor and MockitoBeanOverrideProcessor need to be public.

Related Issues

• Provide a safe way to override and mock beans in the TestContext framework #29917