[WIP] [skip ci] Attempt to allow the use of servant-client in Galley tests

[jschaul]

Currently can't resolve the missing HasClient instance, will need to pair with someone.

Unless you are writing a new backend for servant-client-core or new combinators that you want to support client-generation, you can ignore this class.

Well, apparently one can't ignore this class.

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.
• [ ] The CHANGELOG.md file in the Unreleased section has been updated to explain the change which will be included in the release notes.
• [x] If a component uses a new or changed internal endpoint of another component, this is mentioned in the CHANGELOG.md.
• [x] 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.