Yyakhnytsia/implement missing block kit elements

[yyakhnytsia]

Implement missing elements form the Slack block-kit. Add tests to test the post message with the given payload using real api call with bot token and channel id

[opaliukh]

Three notes:

1. Delete WorkflowIF + WorkflowTriggerIF (dead duplicates). They re-model the same objects as WorkflowObjectIF/TriggerObjectIF, which is what WorkflowButton actually uses — nothing references Workflow. Slack only defines a workflow object + trigger object anyway.
2. Nit: TriggerObjectIF returns ImmutableList<...> — prefer List<...> to match the other models.
3. Nit: @JsonProperty("is_skill_invocation") / @JsonProperty("is_slack_url") are redundant; SnakeCaseStrategy already derives those names.

[yyakhnytsia]

@opaliukh Good catch, fixed.

[yyakhnytsia]

@opaliukh can you take another look at it?