[bitbucket-server] Add Draft to PullRequest payload

Adds `Draft` to `PullRequest`

Object details:
https://confluence.atlassian.com/bitbucketserver/draft-pull-requests-1354498120.html
Support added in:
https://jira.atlassian.com/browse/BSERV-4262

Author: rjepstein

bitbucket-server/payload.go

@@ -220,6 +220,7 @@ type PullRequest struct {
 	FromRef      RepositoryReference      `json:"fromRef"`
 	ToRef        RepositoryReference      `json:"toRef"`
 	Locked       bool                     `json:"locked"`
+	Draft        bool                     `json:"draft"`
 	Author       PullRequestParticipant   `json:"author"`
 	Reviewers    []PullRequestParticipant `json:"reviewers"`
 	Participants []PullRequestParticipant `json:"participants"`