Allow some commands to be run without authenticating against the API

[optik-aper]

Description

These changes re-work some of the authentication logic used to pass the API key when executing commands. Before this, ALL commands authenticated with this key, even the version command. This PR moves the client logic to allow setting a basic HTTP client without bearer tokens. It also moves the missing API key check so that it happens in a pre-Execute check on each top-level command. This check cascades down to all of its children.

Related Issues

closes #359
#332
#92

Testing

• Open a new shell and do not export VULTR_API_KEY
• Test these commands:

• go run main.go account
• go run main.go backups list
• go run main.go bare-metal list
• go run main.go billing history list
• go run main.go block-storage list
• go run main.go database list
• go run main.go dns domain list
• go run main.go firewall group list
• go run main.go instance list
• go run main.go iso list
• go run main.go kubernetes list
• go run main.go load-balancer list
• go run main.go network list
• go run main.go object-storage list
• go run main.go reserved-ip list
• go run main.go script list
• go run main.go snapshot list
• go run main.go ssh list
• go run main.go user list
• go run main.go vpc list
• go run main.go vpc2 list

All of which should show something along the lines of:

Error:
Please export your VULTR API key as an environment variable or add 'api-key' to your config file, eg:
export VULTR_API_KEY='<api_key_from_vultr_account>'

Usage:
  vultr-cli database list [flags]

Aliases:
  list, l

Examples:

        # Full example
        vultr-cli database list

        # Summarized view
        vultr-cli database list --summarize


Flags:
  -h, --help            help for list
  -l, --label string    (optional) Filter by label.
  -r, --region string   (optional) Filter by region.
      --summarize       (optional) Summarize the list output. One line per database.
  -t, --tag string      (optional) Filter by tag.

Global Flags:
      --config string   config file (default is $HOME/.vultr-cli.yaml) (default "/home/michael/.vultr-cli.yaml")

exit status 1

Conversely, these should still return data:

• go run main.go plans list
• go run main.go application list
• go run main.go os list
• go run main.go version

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[github-actions]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v2	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[github-actions]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v2	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[github-actions]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v2	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[github-actions]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v2	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[christhemorse]

Tested all of the suggested commands without exporting my API key and everything worked as expected. Added the API key after and tested several of the same commands with the correct expected result. Checked over the public commands again for good measure as well. Approved!