add sticker type (#653)

* add sticker type

this pr adds the message type 'sticker'.
stickers are handled as normal images
but tagged with the header `Chat-Content: sticker`
it's up to the ui to render these stickers appropriate.

* cargo fmt

src/stock.rs

@@ -109,6 +109,8 @@ pub enum StockMessage {
     MsgLocationDisabled = 65,
     #[strum(props(fallback = "Location"))]
     Location = 66,
+    #[strum(props(fallback = "Sticker"))]
+    Sticker = 67,
 }
 
 impl StockMessage {