Extract common value generator out of BeanDefinitionPropertyValueCodeGenerator

[snicoll]

When writing custom code, it's handy to be able to pass basic types and let something generate a CodeBlock for us. Having to deal with the attribute being null (and then writing null) is also something that should be reusable.

[snicoll]

Perhaps a ValueCodeGenerator interface with a default implementation that can load customizations from the classpath could a good first step. Looking at this need, it matches what #31427 is requesting. Devil in the details if we move this to spring-core I am sure but we'll see how far we can go.