Federation: POST /list-conversations endpoint

[jschaul]

This implements a new endpoint POST /list-conversations similar to the existing GET /conversations but allows specifying parameters, such as a list of remote conversations in its request body. It will also, when called with an empty body, return a user's remote conversations.
This is part of https://wearezeta.atlassian.net/browse/SQCORE-134

todos:

• implement local conversations behaviour
• same test coverage than GET /conversations
• actually make remote calls
• mockFederator test
• end2end test

Checklist

• Title of this PR explains the impact of the change.
• The 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 a component uses a new or changed internal endpoint of another
  component, this is mentioned in the CHANGELOG.md.
• The CHANGELOG.md file in the Unreleased section has been updated to
  explain the change which will be included in the release notes.
• If this PR creates a new endpoint, or adds a new configuration flag, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.

[pcapriotti]

Looks good! Minor comments follow.

EDIT: the end2end test is failing, though. 🤔