Add methods to obtain placeholder names within an expansion

[4drian3d]

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#audiencePlaceholdersByName
• Expansion#relationalPlaceholdersByName
• Expansion#globalPlaceholdersByName
• Expansion#audiencePlaceholdersByNameRaw
• Expansion#relationalPlaceholdersByNameRaw
• Expansion#globalPlaceholdersByNameRaw