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` ?

: SeekableReadBufferWithSize(nullptr, 0)
            , uri {uri_}
            , method {!method_.empty() ? method_ : out_stream_callback_ ? Poco::Net::HTTPRequest::HTTP_POST : Poco::Net::HTTPRequest::HTTP_GET}
            , session {session_}
            , out_stream_callback {out_stream_callback_}
            , credentials {credentials_}
            , http_header_entries {http_header_entries_}
            , remote_host_filter {remote_host_filter_}
            , buffer_size {buffer_size_}
            , use_external_buffer {use_external_buffer_}
            , read_range(read_range_)
            , http_skip_not_found_url(http_skip_not_found_url_)
            , settings {settings_}
            , log(&Poco::Logger::get("ReadWriteBufferFromHTTP"))
        {
            if (settings.http_max_tries <= 0 || settings.http_retry_initial_backoff_ms <= 0
                || settings.http_retry_initial_backoff_ms >= settings.http_retry_max_backoff_ms)
                throw Exception(ErrorCodes::BAD_ARGUMENTS,
                                "Invalid setting for http backoff, "
                                "must be http_max_tries >= 1 (current is {}) and "
                                "0 < http_retry_initial_backoff_ms < settings.http_retry_max_backoff_ms (now 0 < {} < {})",
                                settings.http_max_tries, settings.http_retry_initial_backoff_ms, settings.http_retry_max_backoff_ms);
            if (!delay_initialization)
            {
                initialize();
                if (exception)
                    std::rethrow_exception(exception);
            }
        }
        void call(Poco::Net::HTTPResponse & response, const String & method_)