Add support for configuration property cross-references

[wilkinsona]

#42482 has shown a need for the annotation processor to support cross-referencing in configuration property descriptions. Initially at least, we'd only want to support references where both properties exist in or beneath the same @ConfigurationProperties class.

We'll need to devise a syntax to use for the cross-references. The references will then be automatically replaced with the fully-qualified name of the referenced property during metadata generation. When designing this syntax, we should take support for Markdown-formatted javadoc comments into account.