MM-13826 Webhooks: Allow "true"/"false" for bool values in payload body (#10114)

* Webhooks: Allow "true"/"false" for bool values in payload body

Some slack integrations encode bool fields as "true"/"false", which
was previously unsupported in mattermost due to how encoding/json works.
This commit adds an aliased type for bool that implements json.Unmarshaler
to maintain compatibility with Slack.

* Add missing copyright header to added files

model/slack_attachment.go

@@ -31,9 +31,9 @@ type SlackAttachment struct {
 }
 
 type SlackAttachmentField struct {
-	Title string      `json:"title"`
-	Value interface{} `json:"value"`
-	Short bool        `json:"short"`
+	Title string              `json:"title"`
+	Value interface{}         `json:"value"`
+	Short SlackCompatibleBool `json:"short"`
 }
 
 func StringifySlackFieldValue(a []*SlackAttachment) []*SlackAttachment {