openapi design. client-server mode. let controller to implement interface

[Anilople]

Is your feature request related to a problem? Please describe.

If someone want to find which controller in portal implement the method com.ctrip.framework.apollo.openapi.client.ApolloOpenApiClient#createCluster, it will take some effort.

How about add some interface to make it more easier?

For example:

• interface ApolloXXXOpenApi: in module apollo-openapi, user can use it directly.
• class ApolloXXXOpenApiClient: in module apollo-openapi, the client's implementation of ApolloXXXOpenApi, user cannot access it.
• class ApolloXXXOpenApiController: in module apollo-portal, the server's implementation of ApolloXXXOpenApi

Use this pattern, we can add more openapi faster than before?

Describe the solution you'd like
Keep the compability, add interfaces to module apollo-openapi, let controllers in module apollo-portal implement them.

Describe alternatives you've considered
It is ok to use spring-web in module apollo-openapi? We can maintain the url information in ApolloXXXOpenApi

public interface ApolloNamespaceOpenApi {
  @GetMapping(value = "/openapi/v1/envs/{env}/apps/{appId}/clusters/{clusterName}/namespaces")
  List<OpenNamespaceDTO> findNamespaces(String appId, String env, String clusterName);
}

Then the client and server can use this information, the user also can view it directly.

Drawback: apollo-openapi is a lower level than user's code, so apollo-openapi cannot use spring cause it maybe conflict with user's code