Allow for inheritance of Query Interface

Enhancement request.

## Description:

Currently the clickhouse::Query interface cannot be inherited for any requirements which deviate from the existing implementation.

e.g, if we need a callback for when the query is finished, we should be able to override the Query as follows:
```C++
#ifndef CLICKHOUSE_OVERRIDES
#define CLICKHOUSE_OVERRIDES

#include

namespace DB
{

class Query : public clickhouse::Query
{

public:
Query();
Query(const char *query, const char *query_id = nullptr);
Query(const std::string &query, const std::string &query_id = default_query_id);
Query &OnFinish(std::function callback);

std::function _finish_callback;
void OnFinish() override;

private:
};
}

#endif // CLICKHOUSE_OVERRIDES
```

However this does not work and the original function `OnFinish` of clickhouse::Query is called instead of DB::Query.

The culprit as far as I can understand is:
```C++
// clickhouse/client.cpp
void Client::Impl::ExecuteQuery(Query query) {
EnsureNull en(static_cast(&query), &events_);

if (options_.ping_before_query) {
RetryGuard([this]() { Ping(); });
}

SendQuery(query);

while (ReceivePacket()) {
;
}
}
```

Here the function is called without reference and I believe that the compiler is creating a copy of clickhouse::Query with data from the object passed. This causes the overrides from DB::Query to not be called.