Redesign inner Pointcut implementations as standalone classes

[jhoeller]

In the course of #26266, it turned out to be suboptimal to have Pointcut implementations attached to an enclosing class, e.g. for transaction attribute sources and cache operation sources. It is preferable for the necessary state to be locally concealed within the Pointcut instance.

Also, JCacheOperationSourcePointcut turned out to be accidentally public and will be deprecated as of 6.0.10 now.