Custom KeyGenerator

This commit adds an extra parameter to the base @Cache method
annotations: keyGenerator. This parameter holds the name of the
KeyGenerator bean to use to compute the key for that specific
caching endpoint.

This gives therefore a third way to customize the key. These are:
1. Default KeyGenerator (global for all endpoints)
2. The 'key' attribute of the annotation, giving the SpEL expression to use
3. The 'keyGenerator' attribute of the annotation

The annotation attributes are therefore exclusive. Trying to specify
them both will result in an IllegalStateException.

The KeyGenerator to use for a given operation is cached on startup
so that multiple calls to it does not resolve the instance to use over and
over again.

Issue: SPR-10629
Add custom cache manager per cache operation

It is now possible to specify the CacheManager to use per operation.
The related cache annotation now has an extra attribute that defines
the name of the CacheManager bean to use.  The cache manager that
was previously used is therefore a 'default' cache manager (i.e. the
one to use if no custom cache manager has been set on the operation).

Issue: SPR-8696
polishing