Restructure go/api package

1. Rename `api` into `edl/resource`.

1. Rename function [`api.ConfigureClient`](https://github.com/PaddlePaddle/cloud/blob/dcebcdf820d4a7aa13e815e75d3a17f705794ec1/go/api/register.go#L27) into `edl.resource.RegisterTrainingJob`.

1. Merge `api/register.go` into `api/types.go`, and rename `api/types.go` into `edl/resource/training_job.go`.

  
 