Add support for Vultr

[arunsathiya]

Add support for Vultr. Fixes #158.

• The shell plugin supports importing of API Key from ~/.vultr-cli.yaml file.
• Provisions the credential as an environment variable VULTR_API_KEY

Testing steps

• Sign up for a Vultr account.
• Get an API Token from this page. You'll need to set up billing to get a token though.
• Set up Vultr CLI using vultr-cli account, and create a file at ~/.vultr-cli.yaml with the content: api-key: key-goes-here
• Checkout this branch and run make vultr/build
• Run op plugin init vultr-cli, and source shell plugin alias as well by running source ~/.op/plugins.sh
• Follow on-screen instructions to import your token from the config file.
• Restart shell and run vultr-cli and notice that the credential is now fetched from the 1Password item and provisioned as an environment variable.

Example secret:

➜  shell-plugins git:(add/vultr) make vultr/example-secrets
go run cmd/contrib/main.go vultr/example-secrets
API Key:
  API Key: MH0SBC3BVWH27A00Q9CCCVKJKDUWCEXAMPLE

Thanks and feedback welcome!

[hculea]

Looks great, thank you so much for this addition! ❤️ I've only left a couple of nitpicks.

[arunsathiya]

Thanks for raising some great points, @hculea! I have addressed your feedback above.

[hculea]

Awesome! It now looks good to me, many thanks for this addition!

Code review ✅

[SimonBarendse]

Thank your for another great addition, Arun! 💚