CLI: rewrite defaults

[migmartri]

Currently, the Chainloop CLI has "baked" some defaults for the ControlPlane and Artifact proxy API endpoints.

chainloop 
Chainloop Command Line Interface
...
Flags:
      --artifact-cas string    URL for the Artifacts Content Addressable Storage (CAS) (default "api.cas.chainloop.dev:443")
      --control-plane string   URL for the Control Plane API (default "api.cp.chainloop.dev:443")
...

These options can be overridden on demand by running chainloop config save ... which then stores such overrides in a local config file.

This task is instead about being able to override the defaults so the CLI, once compiled it's ready to use pointing to any custom Chainloop instance.

• Offer optional build options to override both flags
• Document how to compile the CLI binary with custom options