Add methods to obtain placeholder names within an expansion #174
Description
something like:
final Expansion expansion = Expansion.builder("test")
.audienceExpansion("audience", (audience, ctx, queue) -> null)
.build();
final Collection<String> audiencePlaceholdersNames = expansion.audiencePlaceholdersByName();
System.out.println(audiencePlaceholdersNames.get(0));
// "<test_audience>"Expansion#audiencePlaceholdersByNameExpansion#relationalPlaceholdersByNameExpansion#globalPlaceholdersByNameExpansion#audiencePlaceholdersByNameRawExpansion#relationalPlaceholdersByNameRawExpansion#globalPlaceholdersByNameRaw