Improve `toString()` for `AbstractBeanDefinition` and `BeanMetadataAttribute`

[sbrannen]

To improve diagnostics, we should improve the String representations of bean definitions and bean metadata attributes.

The toString() implementation of AbstractBeanDefinition currently includes the bean class name in brackets class [...]; whereas, everything else is not in brackets and just uses an equals sign -- for example, scope=singleton.

With modern Spring applications that make heavy use of @Bean methods in @Configuration classes, bean definitions do not have a pre-configured bean class name. Thus, it makes sense to align the formatting of the bean class name with the formatting of other properties -- for example, class=....

With the recent introduction of @Fallback support, it would also be good to include the fallback flag in toString() alongside the primary flag.

The String representation of a BeanMetadataAttribute could also be improved to include the value in addition to the name.