Refactor mock-client to TS

[spaenleh]

Using the mock-client in typescript is not ideal because there are no type informations available. This leads to cumbersome // @ts-ignore and // eslint-disable-next-line @typescript-eslint/ban-ts-ignore comments.

[mochiruntime]

We may want to move the mock next to its implementation in query-client anyway. However, since we may be using re-using the connection at a higher level (e.g. reused for apps in apps-query-client), we should see if the definition should be lifted up (where???).