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<UpdatablePooledSession>>;
public:
    explicit PooledReadWriteBufferFromHTTP(Poco::URI uri_,
        const std::string & method_ = {},
        OutStreamCallback out_stream_callback_ = {},
        const ConnectionTimeouts & timeouts_ = {},
        const Poco::Net::HTTPBasicCredentials & credentials_ = {},
        size_t buffer_size_ = DBMS_DEFAULT_BUFFER_SIZE,
        const UInt64 max_redirects = 0,
        size_t max_connections_per_endpoint = DEFAULT_COUNT_OF_HTTP_CONNECTIONS_PER_ENDPOINT)
        : Parent(std::make_shared<UpdatablePooledSession>(uri_, timeouts_, max_redirects, max_connections_per_endpoint),
              uri_,
              credentials_,
              method_,
              out_stream_callback_,
              buffer_size_)
    {
    }
};
}