Describe Your Changes

Closes #1962

For context:

• There is no good reason why CLI should only allow installing engines prescribed in .cortexrc's supportedEngines. The check should be done on the server-side.
• This makes it easier to add new engines e.g. upcoming Python vLLM engine feat: vLLM backend #2010. This is because if the user doesn't update his .cortexrc, doing something like cortex engines install vllm will fail otherwise (and we need to add code to somehow update .cortexrc automatically)

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed