[FTS] Create Full Text Index Query Spec

We need a spec for index querying (index management too, but querying has higher priority) which properly defines the officially supported functionality and endpoints. This should include all the query types and their properties, as well as the response block.

On the one hand this helps documenting current behaviour, but also puts us into the mindset that this spec'ed API needs to be considered with all changes going forward, helping to mitigate future breaking changes.

If we have this, the SDK RFC can also stand und much more stable shoulders once it is accepted.

In order of priority I'd say: response, request

Thanks,
Michael