plugin: support protocol dubbo/generic

通过为plugin增加protocol字段，来支持HTTP以外的更多协议。允许的值 Dubbo,Generic,HTTP，默认值为HTTP

apiVersion: microservice.slime.io/v1alpha1
kind: EnvoyPlugin
metadata:
  name: multiproto
spec:
  host:
  - xxx.com:80
  route:
  - xxx.com:80/myroute
  plugins:
  - enable: true
    protocol: Dubbo
    inline:
      settings: {}
    name: xxx
---
apiVersion: microservice.slime.io/v1alpha1
kind: PluginManager
metadata:
  name: multiproto
spec:
  plugin:
  - enable: true
    inline:
      settings: {}
    name: xxx
    port: 8090    
    protocol: Dubbo

slime-io / slime

plugin: support protocol dubbo/generic #429