[Dubbo-3795] replace hard code strings with constants in config moudule

[Duncan-tree-zhou]

What is the purpose of the change

replace hard code strings with constants in config module.

Brief changelog

the following hard code strings had been replaced:

• in ServiceConfig:
  "revision" -> Constants.REVISION_KEY
  "registry" -> Constants.REGISTRY_KEY
• in RegistryConfig:
  "username" -> Constants.USERNAME_KEY
  "password" -> Constants.PASSWORD_KEY
  "file" -> Constants.FILE_KEY
  "client" -> Constants.CLIENT_KEY
• in ReferenceConfig:
  "revision" -> Constants.REVISION_KEY
  "methods" -> Constants.METHODS_KEY
  "client" -> Constants.CLIENT_KEY
• in ProviderConfig:
  "contextpath" -> Constants.CONTEXTPATH_KEY
  "status" -> Constants.STATUS_KEY
• in AbstractInterfaceConfig
  "logstat" -> Constants.LOGSTAT_PROTOCOL
  "local" -> Constants.LOCAL_KEY
  "cluster" -> Constants.CLUSTER_KEY
  "proxy" -> Constants.PROXY_KEY
  "filter" -> Constants.FILE_KEY
  "listener" -> Constants.LISTENER_KEY
  "layer" -> Constants.LAYER_KEY

the following constant vars in Constants.java :

    public static final String STATUS_KEY = "status";
    public static final String CONTEXTPATH_KEY = "contextpath";
    public static final String LISTENER_KEY = "listener";
    public static final String LAYER_KEY = "layer";

    public static final String LOGSTAT_PROTOCOL = "logstat";

Verifying this change

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[Duncan-tree-zhou]

related issue #3795

[ralf0131]

LGTM, thanks for the pull request