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("(", out);
    auto first = true;
    for (size_t i = beg; i < end; ++i)
    {
        if (!first)
            writeString(", ", out);
        first = false;
        auto serialization = (*dict_struct.key)[i].type_serialization;
        serialization->serializeTextQuoted(*key_columns[i], row, out, format_settings);
    }
    writeString(")", out);
}
void ExternalQueryBuilder::composeUpdateCondition(const std::string & update_field, const std::string & time_point, WriteBuffer & out)
{
    writeString(update_field, out);
    writeString(" >= '", out);
    writeString(time_point, out);
    writeChar('\'', out);
}
void ExternalQueryBuilder::composeIdsCondition(const std::vector<UInt64> & ids, WriteBuffer & out) const