Add custom function support

This pull request adds remote function support, which is still in beta, to this SDK. Sorry, the duplicated PRs (due to my operation error) are confusing but this is the final version of https://github.com/slackapi/java-slack-sdk/pull/1239

Developers can implement a remote function and its interactivity this way:

// app.event(FunctionExecutedEvent.class, (req, ctx) -> {
// app.function("hello", (req, ctx) -> {
app.function(Pattern.compile("^he.+$"), (req, ctx) -> {
  ctx.logger.info("req: {}", req);
  ctx.client().chatPostMessage(r -> r
    .channel(req.getEvent().getInputs().get("user_id").asString())
    .text("hey!")
    .blocks(asBlocks(actions(a -> a.blockId("b").elements(asElements(
      button(b -> b.actionId("remote-function-button-success").value("clicked").text(plainText("block_actions success"))),
      button(b -> b.actionId("remote-function-button-error").value("clicked").text(plainText("block_actions error"))),
      button(b -> b.actionId("remote-function-modal").value("clicked").text(plainText("modal view")))
    )))))
  );
  return ctx.ack();
});

app.blockAction("remote-function-button-success", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", req.getPayload().getFunctionData().getInputs().get("user_id").asString());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});
app.blockAction("remote-function-button-error", (req, ctx) -> {
  ctx.client().functionsCompleteError(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .error("test error!")
  );
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});
app.blockAction("remote-function-modal", (req, ctx) -> {
  ctx.client().viewsOpen(r -> r
    .triggerId(req.getPayload().getInteractivity().getInteractivityPointer())
    .view(view(v -> v
      .type("modal")
      .callbackId("remote-function-view")
      .title(viewTitle(vt -> vt.type("plain_text").text("Remote Function test")))
      .close(viewClose(vc -> vc.type("plain_text").text("Close")))
      .submit(viewSubmit(vs -> vs.type("plain_text").text("Submit")))
      .notifyOnClose(true)
      .blocks(asBlocks(input(input -> input
        .blockId("text-block")
        .element(plainTextInput(pti -> pti.actionId("text-action").multiline(true)))
        .label(plainText(pt -> pt.text("Text").emoji(true)))
      )))
    )));
  ctx.client().chatUpdate(r -> r
    .channel(req.getPayload().getContainer().getChannelId())
    .ts(req.getPayload().getContainer().getMessageTs())
    .text("Thank you!")
  );
  return ctx.ack();
});

app.viewSubmission("remote-function-view", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", ctx.getRequestUserId());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  return ctx.ack();
});
app.viewClosed("remote-function-view", (req, ctx) -> {
  Map<String, Object> outputs = new HashMap<>();
  outputs.put("user_id", ctx.getRequestUserId());
  ctx.client().functionsCompleteSuccess(r -> r
    .functionExecutionId(req.getPayload().getFunctionData().getExecutionId())
    .outputs(outputs)
  );
  return ctx.ack();
});

References:

https://github.com/slackapi/bolt-python/pull/986

Category (place an `x` in each of the `[ ]`)

[x] bolt (Bolt for Java)
[ ] bolt-{sub modules} (Bolt for Java - optional modules)
[x] slack-api-client (Slack API Clients)
[ ] slack-api-model (Slack API Data Models)
[ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
[x] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

Files	Patch %	Lines
bolt/src/main/java/com/slack/api/bolt/App.java	57.14%	2 Missing and 4 partials :warning:
...m/slack/api/bolt/request/builtin/EventRequest.java	57.14%	0 Missing and 3 partials :warning:
...ck/api/bolt/request/builtin/ViewClosedRequest.java	33.33%	1 Missing and 1 partial :warning:
.../main/java/com/slack/api/bolt/context/Context.java	83.33%	0 Missing and 1 partial :warning:
...k/api/bolt/request/builtin/BlockActionRequest.java	85.71%	0 Missing and 1 partial :warning:

slackapi / java-slack-sdk

Add custom function support #1241

Category (place an `x` in each of the `[ ]`)

Requirements

Codecov Report

slackapi / java-slack-sdk

Add custom function support #1241

Category (place an x in each of the [ ])

Requirements

Codecov Report

Category (place an `x` in each of the `[ ]`)