fix: allow deserialization of recent link_shared (#260)

`link_shared` events in the Slack developer sandboxes do not contain the
`source` and `unfurl_id` options anymore, as the associated
[`chat.unfurl`](https://api.slack.com/methods/chat.unfurl) API method
now mandates passing the `(channel, ts)` tuple over the
`(source, unfurl_id)` one

> Both channel and ts must be provided together, or unfurl_id and source
must be provided together.

_And_ `channel`, `ts` are marked as required arguments.

Close: #259

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "slack-morphism"
-version = "2.1.1-alpha.0"
+version = "2.2.0-alpha.0"
 authors = ["Abdulla Abdurakhmanov <me@abdolence.dev>"]
 edition = "2021"
 license = "Apache-2.0"

src/models/events/push.rs

@@ -205,8 +205,8 @@ pub struct SlackLinkSharedEvent {
     pub is_bot_user_member: bool,
     pub links: Vec<SlackLinkObject>,
     pub message_ts: SlackTs,
-    pub source: String,
-    pub unfurl_id: SlackUnfurlId,
+    pub source: Option<String>,
+    pub unfurl_id: Option<SlackUnfurlId>,
     pub user: SlackUserId,
 }