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` ?

{
            size_t begin = 0;
            std::optional<size_t> end;
        };
    protected:
        Poco::URI uri;
        std::string method;
        std::string content_encoding;
        UpdatableSessionPtr session;
        std::istream * istr; 
        std::unique_ptr<ReadBuffer> impl;
        std::function<void(std::ostream &)> out_stream_callback;
        const Poco::Net::HTTPBasicCredentials & credentials;
        std::vector<Poco::Net::HTTPCookie> cookies;
        HTTPHeaderEntries http_header_entries;
        RemoteHostFilter remote_host_filter;
        std::function<void(size_t)> next_callback;
        size_t buffer_size;
        bool use_external_buffer;
        size_t offset_from_begin_pos = 0;
        Range read_range;
        
        std::exception_ptr exception;
        bool retry_with_range_header = false;
        
        std::optional<Poco::URI> saved_uri_redirect;
        bool http_skip_not_found_url;
        ReadSettings settings;
        Poco::Logger * log;
        bool withPartialContent() const