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` ?

{
            try
            {
                istr = callImpl(saved_uri_redirect ? *saved_uri_redirect : uri, response, method_);
            }
            catch (...)
            {
                if (response.getStatus() == Poco::Net::HTTPResponse::HTTPStatus::HTTP_NOT_FOUND
                    && http_skip_not_found_url)
                {
                    initialization_error = InitializeError::SKIP_NOT_FOUND_URL;
                }
                else
                {
                    throw;
                }
            }
        }
        /**
         * Throws if error is retriable, otherwise sets initialization_error = NON_RETRIABLE_ERROR and
         * saves exception into `exception` variable. In case url is not found and skip_not_found_url == true,
         * sets initialization_error = SKIP_NOT_FOUND_URL, otherwise throws.
         */
        void initialize()