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.
}
    for (const auto & attr : dict_struct.attributes)
    {
        writeString(", ", out);
        if (!attr.expression.empty())
        {
            writeParenthesisedString(attr.expression, out);
            writeString(" AS ", out);
        }
        writeQuoted(attr.name, out);
    }
    writeString(" FROM ", out);
    if (!db.empty())
    {
        writeQuoted(db, out);
        writeChar('.', out);
    }
    if (!schema.empty())
    {
        writeQuoted(schema, out);
        writeChar('.', out);
    }
    writeQuoted(table, out);
    if (!where.empty())
    {
        writeString(" WHERE ", out);
        writeString(where, out);
    }
}
std::string ExternalQueryBuilder::composeUpdateQuery(const std::string & update_field, const std::string & time_point) const