BE: Retrieve labels of a Jira issue on details page

Description

This is related to the other backend issue #333127 (closed) and also corresponds with the frontend issue #328787 (closed).

We get all the Jira Issue data in one request using the jira-ruby gem (jira_service.find_issue).

In this case, we want to be able to modify the labels and update the new labels in Jira, we have to do something like:

1. Make a request (or several having pagination) to Jira in order to pull the available labels. Caveat: Only available for Jira cloud, see #293021 (comment 551848967).
2. Populate the UI with the available labels to be selected by the user #328787 (closed).