feat: Support getting PR linked JIRA issues

[hrak]

When Bitbucket is configured with JIRA integration, the issues linked to a PR (either by mentioning in commit msgs, branch name or PR title/description) can be retrieved from the following endpoint:

/rest/jira/1.0/projects/<project slug>/repos/<repo slug>/pull-requests/<PR ID>/issues

The response looks like this:

[{"key":"CC-711","url":"https://jira.example.com/browse/CC-711"}]

It would be great if this library supported this.