Support Maps in ReflectiveConfigGroup

[marecabo]

This change allows to store Maps of primitive Java types within ReflectiveConfigGroup using the @Parameter annotation and closes #2815

Keys will always be quoted and properly XML-excaped when written to the config file.

Example

@Comment("map of LocalDate")
@Parameter
public Map<Character, LocalDate> localDateMapField = Map.of('d', LocalDate.of(2023, 10, 9));

will result in

<!-- map of LocalDate -->
<param name="localDateMapField" value="{&quot;d&quot;:[2023,10,9]}" />

[michalmac]

This PR is technically correct and looks good to me. I just wonder if this feature will be used often enough to justify the additional complexity of the code. Especially, the need of escaping (e.g. value="{"d":[2023,10,9]}") may be discouraging.