New abstraction for token generation

[sky-code]

Instead of REST_USE_JWT flag introduce a new abstraction for token generation (token provider)
djoser as example already have TOKEN_MODEL settings parameter for this.
This is required for adding new token providers as example this pull request for knox tokens support, add another settings flag REST_USE_KNOX , this flags can be replaced by one settings parameter token_provier or token_model or token_generator one settings parameter instead of many flags.