New support on Service/Server class

[wonkr]

Our current service layer design can only generate one type of server. There is a service that might need multiple types of server to install. Some of them requires mandatory server. (i.e. ChainLinkService)

[wonkr]

For ChainlinkService, it requires one initializerServer to operate chainlink Servers. So there are two types of server in this service; initializer and ChainlinkService.
In this case, we decided to add two methods inside the ChainlinkService class as below.

class ChainlinkService:
      def installInitializer(self, vnode:str) -> ChainLinkInitializerServer:
         if vnode in self._pending_targets.keys(): 
               return    self._pending_targets[vnode]

          s = self._createInitializerServer()
          self._pending_targets[vnode] = s

          return self._pending_targets[vnode]
       def _createInitializerServer(self) -> ChainlinkInitializerServer:
           ...

User code will be as below

# initializer
chainlink.installInitializer(cnode)
# normal server
chainlink.install(cnode)

[wonkr]

Need to think about extending this design to the skeleton code.