Harmonize configuration properties that accept a comma-separated list of values

[snicoll]

Looking at #42472, I found odd that we'd use a String for a property that's declared as a "comma-separated list of". Then I noticed that MessageSourceProperties has the same pattern for the basename property.

Looking at our code base, most of these properties are a List or an array, with a few exceptions. I am wondering if we shouldn't harmonize this across the code base so that it's always a collection.

[wilkinsona]

I had the same thought when briefly looking at #42472.

Consistently using List or similar sounds good to me, particularly where the target of the property is a collection-like type and we're having to do something like StringUtils.commaDelimitedListToStringArray when setting it.

[philwebb]

I think this code probably pre-dates the binder rewrite so +1 to moving to a List<String>. That will also allow the YAML list syntax to work.

[wilkinsona]

As part of this, I'm going to revise the property descriptions to just say "list" rather than "comma-separated list". My hope is that this will make it a bit more clear that YAML's list syntax can be used.