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.
auto first = true;
        for (const auto & key : *dict_struct.key)
        {
            if (!first)
                writeString(", ", out);
            first = false;
            if (!key.expression.empty())
            {
                writeParenthesisedString(key.expression, out);
                writeString(" AS ", out);
            }
            writeQuoted(key.name, out);
        }
        if (dict_struct.range_min && dict_struct.range_max)
        {
            writeString(", ", out);
            if (!dict_struct.range_min->expression.empty())
            {
                writeParenthesisedString(dict_struct.range_min->expression, out);
                writeString(" AS ", out);
            }
            writeQuoted(dict_struct.range_min->name, out);
            writeString(", ", out);
            if (!dict_struct.range_max->expression.empty())
            {
                writeParenthesisedString(dict_struct.range_max->expression, out);
                writeString(" AS ", out);
            }
            writeQuoted(dict_struct.range_max->name, out);
        }