Add app spec validate-offline (Resolves #1449)

[trinitronx]

This PR implements a new sub-command, as described in #1449:

doctl  apps spec validate-offline

I followed a similar method to #225

When merged, this enables the following GitHub Actions feature to work:

• Add no auth option trinitronx/action-doctl#1
  • Note: this is a draft PR against my fork to demonstrate the changes
  • Fixes Setup without token for offline commands such as app spec validation action-doctl#67

Notes: My work on this PR was done as part of DigitalOcean + MLH's Hacktoberfest + Global Hack Week: Open Source

[andrewsomething]

Thanks so much for this great contribution!

You provided a lot of great context in the issue. I also prefer your original design of the --schema-only flag being used to determine if the token is needed or not. Reading over the problems you ran into, you are right that the --schema-only flag is not available when command creation happens. But we can workaround this!

We can call initServices directly if needed:

	if err := c.initServices(c); err != nil {
		return err
	}

So rather than adding a new sub-command we can use the --schema-only flag for this by:

1. Creating the command with cmdBuilderWithInit like you did with the validate-offline one.
2. Between the if schemaOnly block and the c.Apps().Propose call, add the call to initServices

This way, if the --schema-only flag is passed, we exit before initializing the services. If not, we still initialize the services and check for the API token before making the API call.

[trinitronx]

Thanks so much for this great contribution!

You provided a lot of great context in the issue. I also prefer your original design of the --schema-only flag being used to determine if the token is needed or not. Reading over the problems you ran into, you are right that the --schema-only flag is not available when command creation happens. But we can workaround this!

We can call initServices directly if needed:

[...SNIP...]

@andrewsomething: Thanks for that suggestion! I've refactored the code to use that method, and tested it against some proposed changes to action-doctl. It's now tested & validated as working!

This is ready for review again 😀

P.S.: 🙏 If someone could add the Hacktoberfest issue labels once this is accepted, that would be great! Thanks!

[andrewsomething]

👍 LGTM! Thanks again!