request help: support get cookies method in request interface

### Issue description
In our plugin implementation we need get cookies from request, we can get cookie from header by
```go
request.Header().Values("Cookie")
```
after that we need to parse the string to cookie structure, however it is not convenient and effective when we have many plugins have to use cookie.

So how about consider to add and implement cookie method in Request interface? like net/http/request.go
```go
Cookies(filter string) []*Cookie
```

### Environment

* APISIX Go Plugin Runner's version: 0.5.0