add 'draft' state for pull-requests

[gorohovart]

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What's in this PR? I'm adding a new state of Pull-Requests: DRAFT

Why this would close this community issue: Differentiate between a draft pull request and a normal pull request in workflow triggers

How has this been tested?
I've added a unit test

[rachellerathbone]

Hey @gorohovart. Thanks for attempting to do this. Unfortunately, it's not so simple. Jira doesn't currently doesn't recognise the PR status of 'DRAFT' and there are a number of services that need to be updated (all within the Atlassian infrastructure) in order for this to be supported.