feat: update destination add to use api endpoint

[mxyng]

Summary

• Use api.{baseDomain} when templating destination add command
• Use values file instead of --set
• Use upgrade --install instead of install
• Make baseDomain required when signup is enabled

[dnephin]

The Go changes LGTM, but I notice there is one test failing that needs to be updated for this new validation.

I guess using a heredoc with a values.yaml file should make the command easier to copy.

[dnephin]

JSON is valid yaml, so we could potentially avoid this dependency by using JSON.stringify() and writing it out as JSON.

[mxyng]

We could do that but IMO JSON is less user friendly with its quotes and braces and brackets. I don't think it's worth using JSON solely to avoid this dependency.

[BruceMacD]

Can we just do this all with flags? The cat piping to get multi-line input looks confusing imo

[mxyng]

We could. It would look like the previous command snippet. IMO it's actually harder to read and understand. Flags simulate a hierarchy which can easily be overlooked. This makes the relationship between the values clear while also making it easier to edit or append.

Additionally, the settings doesn't translate directly to a values file so if the user wants to adds the outputs to IAC, they'll need to transcribe it into YAML. This way, we can add an option to download the file instead of using a heredoc and the helm command will remain the same.