Use correct URL for Studio Prem Environments

[julieg18]

Currently, we use https://studio.iterative.ai for making calls to Studio but self-hosted instances would use a different URL.

We can use the DVC config's studio.url to get the URL to call to and default to https://studio.iterative.ai if it doesn't exist.

• Use studio.url for calls to Studio, defaulting to https://studio.iterative.ai
• Add commands for updating the URL in the command pallette
• Look into adding reference to studio prem URL in setup section