Add `up space list` command

[RedbackThomson]

Description of your changes

Add a new command up space list to the CLI to determine what spaces are available to the current authenticated user.

Example:

$ up space list -a upbound-papper-cutters
NAME                    MODE        PROVIDER   REGION
upbound-legacy-gke      legacy      gcp        us-central-1
upbound-gke-us-west-1   managed     gcp        us-west-1
upbound-aws-us-east-1   managed     aws        us-east-1
attached-pnx7v          connected              

Relies on https://github.com/upbound/up-sdk-go/pull/71

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested