Add qualified broadcast endpoint

[pcapriotti]

This adds a qualified version of the message broadcast endpoint, with the same API as the one for normal messages. This makes it possible for the clients to uniformly use the qualified API for all possible message types.

Tracked by https://wearezeta.atlassian.net/browse/SQCORE-1257.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[mdimjasevic]

I'm approving, but before merging please take a look at the comment on changing [(UserId, (ClientId, Text)] to [(UserId, ClientId, Text)].