Use of custom query/placeholder for external dictionaries.
The `placeholder` is similar to `update_field`

Current implementation supports only simple table field like 
`SOURCE(CLICKHOUSE(... update_field 'added_time' update_lag 15))`
but miss support for complex/compound expressions (that for example could use index more efficiently).

Example: 
query = "
    ....
    from t1
    where :placeholder between col1 and col2 and col3=42
    "
invalidate_query=
placeholder_query = "select max(col1) from t1 ...",
placeholder_init_str = "toDateTime('1970-01-01')"

So initial value of placeholder is calculated from `placeholder_init_str`,
next value from `invalidate_query`
and could be used in query.

By introducing this feature work with dictionary sources will be more flexible and general.
writeString(" WHERE ", out);
        composeUpdateCondition(update_field, time_point, out);
        writeChar(';', out);
        return out.str();
    }
    else
    {
        writeString(query, out);
        auto condition_position = query.find(CONDITION_PLACEHOLDER_TO_REPLACE_VALUE);
        if (condition_position == std::string::npos)
        {
            writeString(" WHERE ", out);
            composeUpdateCondition(update_field, time_point, out);
            writeString(";", out);
            return out.str();
        }
        WriteBufferFromOwnString condition_value_buffer;
        composeUpdateCondition(update_field, time_point, condition_value_buffer);
        const auto & condition_value = condition_value_buffer.str();
        auto query_copy = query;
        query_copy.replace(condition_position, CONDITION_PLACEHOLDER_TO_REPLACE_VALUE.size(), condition_value);
        return query_copy;
    }
}
std::string ExternalQueryBuilder::composeLoadIdsQuery(const std::vector<UInt64> & ids) const
{
    if (!dict_struct.id)
        throw Exception(ErrorCodes::UNSUPPORTED_METHOD, "Simple key required for method");
    WriteBufferFromOwnString out;
    if (query.empty())