Add Completion result and task type to the Inference API spec

[timgrein]

This PR adds the new completion task type and the result to the specification.

Completion result example:

{ 
  "completion": [ 
     { 
       "result": "..." 
      } 
   ] 
} 

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
inference.delete_model	⚪	Missing test	Missing test
inference.get_model	🟢	1/1	1/1
inference.inference	⚪	Missing test	Missing test
inference.put_model	⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.

[davidkyle]

LGTM