Change CLI assign behavior

[aguschin]

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?