Add draft field for bitbucket PR events

Similar to github and gitlab PR events, bitbucket also supports the `draft` field in the payload.
See https://support.atlassian.com/bitbucket-cloud/docs/event-payloads/#Pull-request

This adds the `draft` field, and exposes it analogously to github/gitlab as `Draft`.

Author: finkandreas

bitbucket/payload.go

@@ -470,6 +470,7 @@ type PullRequest struct {
 	Title       string `json:"title"`
 	Description string `json:"description"`
 	State       string `json:"state"`
+	Draft       bool   `json:"draft`"
 	Author      Owner  `json:"author"`
 	Source      struct {
 		Branch struct {