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` ?

{
            if (initialization_error == InitializeError::SKIP_NOT_FOUND_URL)
                return false;
            assert(initialization_error == InitializeError::NONE);
            if (next_callback)
                next_callback(count());
            if (read_range.end && getOffset() == read_range.end.value())
                return false;
            if (impl)
            {
                if (use_external_buffer)
                {
                    /**
                    * use_external_buffer -- means we read into the buffer which
                    * was passed to us from somewhere else. We do not check whether
                    * previously returned buffer was read or not (no hasPendingData() check is needed),
                    * because this branch means we are prefetching data,
                    * each nextImpl() call we can fill a different buffer.
                    */
                    impl->set(internal_buffer.begin(), internal_buffer.size());
                    assert(working_buffer.begin() != nullptr);
                    assert(!internal_buffer.empty());
                }
                else
                {
                    /**
                    * impl was initialized before, pass position() to it to make
                    * sure there is no pending data which was not read.
                    */
                    if (!working_buffer.empty())