feat: add --dry-run arg

[robotrory]

Adds a --dry-run arg so that we can test validation without making real network calls.

[samstarling]

This looks good! One thing to consider is whether we want to split up "global" options from everything else. Now that *CLI gets passed into Run(), BuildPushCmd gets access to things like Auth which ideally it shouldn't need to know anything about.

I think we could solve this by doing something like:

type Globals struct {
    Debug  bool
    JSON   bool
    Quiet  bool
    DryRun bool
}

type CLI struct {
    Globals
    Version kong.VersionFlag
    Auth    AuthCmd  `cmd:""`
    Build   BuildCmd `cmd:""`
}

// Then in main:
kong.Parse(&cli, kong.Vars{...}, kong.Bind(&cli.Globals))

// And in subcommands:
func (c *BuildPushCmd) Run(g *Globals) error { ... }

Not a must-fix by any means, but probably nice to do as we scale the number of commands in here.