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.
{
    switch (quoting_style)
    {
        case IdentifierQuotingStyle::None:
            writeString(s, out);
            break;
        case IdentifierQuotingStyle::Backticks:
            writeBackQuotedString(s, out);
            break;
        case IdentifierQuotingStyle::DoubleQuotes:
            writeDoubleQuotedString(s, out);
            break;
        case IdentifierQuotingStyle::BackticksMySQL:
            writeBackQuotedStringMySQL(s, out);
            break;
    }
}
std::string ExternalQueryBuilder::composeLoadAllQuery() const