Add a new endpoint for auto completing filter paths

[timfdev]

We need a new endpoint to query for unique paths. Example response:

{
  "prefix": "subscription.st",
  "paths": [
    {
      "path": "subscription.status",
      "type": "string",
      "operators": ["eq", "ne", "is_null", "is_not_null"],
      "valueSchema": {
        "eq": { "kind": "string" },
        "ne": { "kind": "string" },
        "is_null": { "kind": "none" },
        "is_not_null": { "kind": "none" }
      },
      "example_values": ["active", "disabled"]
    },
}

We can use this in the frontend to build filters in addition to our existing hybrid search on text.