Support sky.serve.core.up as a library function

[fozziethebeat]

Support sky.serve.core.up as a library function we can call similar to sky.launch.

The method already does all the core work but doesn't return the created endpoints so it's a bit difficult to rely on the results programmatically.

I think all that's needed at a minimum is to return the endpoint result with potentially a bit more for other purposes.

[fozziethebeat]

Thanks for merging my PR!