Add support for radial search on Neural query

[Xtansia]

Description

Add support for radial search on k-NN and Neural query types

Issues Resolved

Fixes #1226

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[dblock]

I see the parameters present in the API spec. How far are we from getting this via the code generator?

[Xtansia]

I see the parameters present in the API spec. How far are we from getting this via the code generator?

The query types (plus a couple others) have special deserialization logic for handling the fact they're "single key dictionaries", keyed on field, and in Java (and .NET) the field value gets flattened into the same class as the other query properties. So requires tweaking the spec to be consistent in how they're represented to be able to add special handling to the generator. It is one of the next things I'm going to look into, and will need to make sure the spec changes work for the people working on protobuf representation of the search api.