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` ?

{
    using Parent = detail::ReadWriteBufferFromHTTPBase<std::shared_ptr<UpdatableSession>>;
public:
        ReadWriteBufferFromHTTP(
        Poco::URI uri_,
        const std::string & method_,
        OutStreamCallback out_stream_callback_,
        const ConnectionTimeouts & timeouts,
        const Poco::Net::HTTPBasicCredentials & credentials_,
        const UInt64 max_redirects = 0,
        size_t buffer_size_ = DBMS_DEFAULT_BUFFER_SIZE,
        const ReadSettings & settings_ = {},
        const HTTPHeaderEntries & http_header_entries_ = {},
        Range read_range_ = {},
        const RemoteHostFilter & remote_host_filter_ = {},
        bool delay_initialization_ = true,
        bool use_external_buffer_ = false,
        bool skip_not_found_url_ = false)
        : Parent(std::make_shared<UpdatableSession>(uri_, timeouts, max_redirects),
            uri_, credentials_, method_, out_stream_callback_, buffer_size_,
            settings_, http_header_entries_, read_range_, remote_host_filter_,
            delay_initialization_, use_external_buffer_, skip_not_found_url_)
    {
    }
};
class UpdatablePooledSession : public UpdatableSessionBase<PooledHTTPSessionPtr>