Add support for hex representation for mixed tensors in queries

[jobergum]

We support hex format in document JSON but not for queries.

{
    "put": "id:doc:doc::1",
    "fields": {
        "text": "To transport goods on water, use a boat",
        "embedding": {
            "0": "3DE38E393E638E393EAAAAAB",
            "1": "3EE38E393F0E38E43F2AAAAB",
            "2": "3F471C723F638E393F800000"
        }
    }
}

Is valid, but attempting to send this format in queries will throw a 400 bad request

vespa query 'yql=select * from doc where true' 'ranking=full' 'input.query(qt)={"0":"3DE38E393E638E393EAAAAAB"}'

{ "errors": [
            {
                "code": 3,
                "summary": "Illegal query",
                "message": "Could not set 'ranking.features.query(qt)' to '{\"0\":\"3DE38E393E638E393EAAAAAB\"}': Could not parse '{\"0\":\"3DE38E393E638E393EAAAAAB\"}' as a tensor of type tensor<float>(querytoken{},v[3]): At value position 0: Expected a '[' but got '\"'"
            }
]}