Support for nested path using field access

This commit adds a nested path support for DirectFieldAccessor that is
similar to what BeanWrapper provides. It is now possible to use
expressions such as "person.address.city.name" to access the name of
the city that a given person lives in using fields to traverse the
graph.

DirectFieldAccessor also now supports an auto-grow option to create
a default instance for a "null" intermediate path. This option is
false by default and leads to a NullValueInNestedPathException in such
a case.

This commit also harmonizes part of the tests suite so that core tests
are shared between BeanWrapperImpl and DirectFieldAccessor.

Note that map and list access is not implemented as part of this
commit.

Issue: SPR-9705