request help: support get cookies method in request interface

[ZhangShangyu]

Issue description

In our plugin implementation we need get cookies from request, we can get cookie from header by

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

Cookies(filter string) []*Cookie 

Environment

• APISIX Go Plugin Runner's version: 0.5.0