Rename `OverrideMetadata` for Bean Overrides

[sbrannen]

Within the org.springframework.test.context.bean.override package, OverrideMetadata sticks out like a sore thumb: it's the only public type whose name does not start with BeanOverride. Considering that alone, I'd suggest we rename it to BeanOverrideMetadata.

However, an OverrideMetadata component plays two roles: a factory for a bean override instance and a holder for metadata, and I would argue that "creation of the bean override instance" is the primary role of such a component. In addition, I find it a bit unusual that a "metadata" component "creates" something. Rather, it's usually the other way around: a component typically creates something from metadata. Along those lines, I consider an OverrideMetadata component more like a factory which creates a bean override instance based on metadata that is has been supplied.

In light of the above, I suggest that we rename OverrideMetadata to BeanOverrideFactory.

[simonbasle]

After discussing this within the team, we aligned on BeanOverrideHandler. We also decided to rework the naming in the API a little bit where createOverride and track would be renamed to createOverrideInstance and trackOverrideInstance.