Consider adding @Delimiter support to String converters

[philwebb]

Spring Boot currently includes a DelimitedStringToArrayConverter and DelimitedStringToCollectionConverter which allows a @Delimiter annotation to be used to control how Strings are converted.

This functionality is quite useful and it might be a good candidate to port to Spring Framework. AFICT we can rework the existing String converters to support it in a back compatible manner.

[rstoyanchev]

We discussed this, and there are a couple of follow-ups needed to help move this forward.

One, is to have another look at all other similar annotations in Boot, some of which may have been added since, and see if this is the only one still to consider.

Two, it would be useful to consider use cases. There is a concrete example in the linked spring-projects/spring-boot#15019 (comment) but I would need to experiment with it. Generally it's not very clear that this is very common in data binding where the Servlet API already breaks each parameter into a Collection of values. A more likely use case could be the use of property placeholders and @Value but we'll need a small POC to see how it works out.