Make skypilot optional for inference-cli

[sberan]

Description

Skypilot is a fairly large dependency of inference-cli that is not frequently used by most users. In this PR, I am adding it as an optional dependency and I added error messages to guide the user toward installing these dependencies if needed.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

Tested locally

Any specific deployment considerations

n/a

Docs

• Docs updated? What were the changes:

[bigbitbus]

Lgtm!