CMS-1701 Add type definitions for contentType_list in RemoteService.ts

Add strict typing for the parameter and result values of RemoteServiceInterface.contentType_list()

* Check the RpcHandler method in Java to find the exact values accepted and returned.
* Use optional types (optParam?: string) where appropriate.
* Avoid use of any as much as possible and use concrete types.
* Look at types of space_list() and space_get() for reference.
Merge pull request #254 from enonic/CMS-1701

CMS-1701 Add type definitions for contentType_list in RemoteService.ts