fix `.add_issues_to_epic()`

[adehad]

Summary of changes

1. Use the correct request method
2. Support List[str] too
3. fix the API url

Docs

https://developer.atlassian.com/cloud/jira/software/rest/api-group-epic/#api-rest-agile-1-0-epic-epicidorkey-issue-post

https://docs.atlassian.com/jira-software/REST/7.3.1/#agile/1.0/epic-moveIssuesToEpic