Add topic field to ConversationIF

[McrsftHater]

A follow-up to this issue: #205

There is a "topic" field missing in ConversationIF

Suggestion

Add to ConversationIF:

@JsonProperty("topic")
Optional<Topic> getTopic();

Add TopicIF interface:

@HubSpotStyle
@JsonNaming(PropertyNamingStrategy.SnakeCaseStrategy.class)
public interface TopicIF {
    @JsonProperty("value")
    Optional<String> getValue();
    @JsonProperty("creator")
    Optional<String> getCreator();
    @JsonProperty("last_set")
    Optional<Long> getLastSet();
}

P.S. There are more fields missing but my project require only this one.