Some URLs expect a User-Agent header.

Example (version 22.2.1.3767):
```sql
SELECT * FROM url('https://api.github.com/users/clickhouse','JSONAsString');
```
Results in:
```
Code: 86. DB::Exception: Received error from remote server /users/clickhouse. HTTP status code: 403 Forbidden, body: 
Request forbidden by administrative rules. Please make sure your request has a User-Agent header (http://developer.github.com/v3/#user-agent-required). Check https://developer.github.com for other possible causes.
: While executing URL. (RECEIVED_ERROR_FROM_REMOTE_IO_SERVER) (version 22.2.1.3767)
```

Maybe it's nice to always send a header along, something like `User-Agent: ClickHouse/22.2.1.3767` ?

impl->position() = position();
                }
            }
            bool result = false;
            size_t milliseconds_to_wait = settings.http_retry_initial_backoff_ms;
            for (size_t i = 0; i < settings.http_max_tries; ++i)
            {
                try
                {
                    if (!impl)
                    {
                        initialize();
                        if (initialization_error == InitializeError::NON_RETRIABLE_ERROR)
                        {
                            assert(exception);
                            break;
                        }
                        else if (initialization_error == InitializeError::SKIP_NOT_FOUND_URL)
                        {
                            return false;
                        }
                        if (use_external_buffer)
                        {
                            
                            impl->set(internal_buffer.begin(), internal_buffer.size());
                            assert(working_buffer.begin() != nullptr);
                            assert(!internal_buffer.empty());
                        }
                    }
                    result = impl->next();