This commit introduces two sets of annotations (`@TestBean` on one side
and `MockitoBean`/`MockitoSpyBean` on the other side), as well as an
extension mecanism based on the `@BeanOverride` meta-annotation.
Extension implementors are expected to only provide an annotation,
a BeanOverrideProcessor implementation and an OverrideMetadata subclass.
Closes gh-29917.
e1bbdf0913