Remove orgMsp from RestApiClientConfig, make user creds non-mandatory in gcclient

Currently `gcclient` allows to connect to GC gateway with `forApiConfig` method, which requires the following parameters:

```
export interface RestApiClientConfig {
orgMsp: string;
apiUrl: string;
userId?: string;
userSecret?: string;
configPath: string;
}
```

But, for REST API we no longer need `orgMsp`, it should be removed.

Additionally `userId` and `userSecret` are related with the legacy setup and they should not be mandatory (they are in `gcclient`). It should be possible to call chaincodes without those values.

TODO:
- [ ] remove `orgMsp` from `RestApiClientConfig` and related
- [ ] mark `userId` and `userSecret` non-required in `gcclient`; instead throw an exception if they are missing on authorizing FabloREST client