Update `SharedMetadataReaderFactoryContextInitializer` to optionally
support a `ConfigurationClassPostProcessor` provided via an instance
`Supplier`.

If the instance supplier is available then we can use that to set the
`MetadataReaderFactory` non-reflectively, instead of using a
`PropertyValue` (and the `BeanWrapper` eventually).

The previous behavior is left as a  fallback and without a change in
Spring Framework will continue to be the default for most applications.

See gh-22858