add slack atd bindings for url verification and chat unfurl

Link unfurling requires usage of Slack's Events API. The first
step in configuring it is url verification. Subsequently, we can
start accepting event callback notifications, one of which is
chat_unfurl.

Author: yasunariw

lib/slack.atd

@@ -1,3 +1,5 @@
+type 'v map_as_object <ocaml from="State"> = abstract
+
 type message_field = {
   ?title: string nullable;
   value: string;
@@ -65,3 +67,53 @@ type post_message_res = {
   ?channel: string nullable;
   ?error: string nullable;
 }
+
+type link_shared_link = {
+  domain: string;
+  url: string;
+}
+
+type link_shared_event = {
+  channel: string;
+  is_bot_user_member: bool;
+  user: string;
+  message_ts: string;
+  ?thread_ts: string option;
+  links: link_shared_link list;
+}
+
+type event = [
+| Link_shared <json name="link_shared"> of link_shared_event
+] <ocaml repr="classic"> <json adapter.ocaml="Atdgen_runtime.Json_adapter.Type_field">
+
+type event_callback_notification = {
+  token: string;
+  team_id: string;
+  api_app_id: string;
+  event: event;
+  event_id: string;
+  event_time: int;
+}
+
+type url_verification_notification = {
+  token: string;
+  challenge: string;
+}
+
+type event_notification = [
+| Event_callback <json name="event_callback"> of event_callback_notification
+| Url_verification <json name="url_verification"> of url_verification_notification
+] <ocaml repr="classic"> <json adapter.ocaml="Atdgen_runtime.Json_adapter.Type_field">
+
+type unfurl = message_attachment
+
+type chat_unfurl_req = {
+  channel: string;
+  ts: string;
+  unfurls: unfurl map_as_object;
+}
+
+type chat_unfurl_res = {
+  ok: bool;
+  ?error: string option;
+}