Add conditional processing option for unhelpful FactoryBeans

OnBeanCondition has some issues with FactoryBean object types where
the FactoryBean is not generic (i.e. you have to instantiate it to
get its object type). This is a known issue (see tests in
ConditionalOnMissingBeanTests), but we can provide some help for
library authors who know the type in advance. The approach we have
taken here is to check the BeanDefinition for an attribute called
"factoryBeanObjectType" (OnBeanCondition.FACTORY_BEAN_OBJECT_TYPE)
which, if it exists, can be used as a tie-breaker. Its value should
be a Class<?> instance.

Fixes gh-921