[feature] direct dubbo invoke

[ztelur]

What this PR does:

• handle http request to dubbo provider with direct generic call using route and cluster
• add direct samples

pixiu config.yml

      filter_chains:
          filters:
            - name: dgp.filter.httpconnectionmanager
              config:
                route_config:
                  routes:
                    - match:
                        prefix: "/UserService"
                      route:
                        cluster: "user"
                http_filters:
                  - name: dgp.filter.http.directdubboproxy
                    config:
  clusters:
    - name: "user"
      lb_policy: "lb"
      endpoints:
        - id: 1
          socket_address:
            address: 127.0.0.1
            port: 20000

http post request

	url := "http://localhost:8883/UserService/com.dubbogo.pixiu.UserService/GetUserByName"
	data := "{\"types\":\"string\",\"values\":\"tc\"}"

[codecov-commenter]

Codecov Report

Merging #434 (7206f8a) into develop (a274007) will decrease coverage by 0.02%.
The diff coverage is 25.00%.

@@             Coverage Diff             @@
##           develop     #434      +/-   ##
===========================================
- Coverage    37.67%   37.65%   -0.03%     
===========================================
  Files           55       55              
  Lines         3740     3745       +5     
===========================================
+ Hits          1409     1410       +1     
- Misses        2174     2177       +3     
- Partials       157      158       +1     

Impacted Files	Coverage Δ
pkg/client/dubbo/dubbo.go	18.83% <ø> (ø)
pkg/server/adapter_manager.go	63.33% <0.00%> (-7.04%)	⬇️
pkg/filter/metric/metric.go	34.21% <100.00%> (ø)
pkg/server/cluster_manager.go	38.46% <0.00%> (+0.16%)	⬆️

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