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.
WriteBuffer & out, size_t beg, size_t end) const
{
    composeKeyTupleDefinition(out, beg, end);
    writeString(" IN (", out);
    bool first = true;
    for (const auto row : requested_rows)
    {
        if (!first)
            writeString(", ", out);
        first = false;
        composeKeyTuple(key_columns, row, out, beg, end);
    }
    writeString(")", out);
}
void ExternalQueryBuilder::composeKeyTupleDefinition(WriteBuffer & out, size_t beg, size_t end) const
{
    if (!dict_struct.key)
        throw Exception(ErrorCodes::UNSUPPORTED_METHOD, "Composite key required for method");
    writeChar('(', out);
    auto first = true;
    for (size_t i = beg; i < end; ++i)
    {
        if (!first)
            writeString(", ", out);
        first = false;
        writeQuoted((*dict_struct.key)[i].name, out);
    }
    writeChar(')', out);
}
void ExternalQueryBuilder::composeKeyTuple(const Columns & key_columns, size_t row, WriteBuffer & out, size_t beg, size_t end) const