Change CLI `assign` behavior

When a Data Scientist assigns a stage, he doesn't operate on REFs, he operates on versions. Thus it should be
```
$ gto assign mymodel VERSION STAGE
```
and for simplicity, we may remove REF from there.

^ Suggested by @mnrozhkov.

Few notes from myself:
- Allowing to specify REF and VERSION causes confusion in GTO CLI. With only one selected CLI commands would be somewhat cleaner.
- This contradicts a direction to be "like Git"
- For models and DS it may work, but GTO tries to be "abstract". Is it worth to remove REF from there?